Counting a Baby's Days and Months: When Are Oshichiya, the 100-Day Celebration and the Half Birthday?
Oshichiya, Omiyamairi, Okuizome. Many of a Japanese baby's milestone ceremonies are set by "day X after birth", but whether you count the birthday as day 1 or day 0 shifts things by a day. This article sorts out the counting rule, the date of each ceremony and how a baby's age in months advances.
For ceremonies, "day X after birth" counts the birthday as day 1
The day counts used for a baby's ceremonies in Japan customarily treat the day of birth as day 1. Oshichiya, held on day 7, falls six days after the birth. Meanwhile, many tools that calculate elapsed days count the birthday as day 0, and that's where the one-day gap comes from.
Here are the dates of the main ceremonies for a baby born on April 10, 2026.
| Ceremony | How it's counted | Born April 10 |
|---|---|---|
| Oshichiya (naming ceremony) | Day 7 after birth | April 16 |
| Omiyamairi (first shrine visit, boys) | Day 31 after birth | May 10 |
| Omiyamairi (girls) | Day 32 after birth | May 11 |
| Okuizome (100-day first-meal celebration) | Day 100 after birth | July 18 |
| Half birthday | 6 months old | October 10 |
| First birthday | 1 year old | April 10, 2027 |
The day count for Omiyamairi varies by region, and some areas hold it for girls on day 33 rather than day 32. Okuizome is usually on day 100, but some regions hold it on day 110 or 120. Asking about your own and your partner's family customs first means you can settle the dates in one conversation.
Month age advances by "the same date", not by days
"One month old" isn't 30 days later. Each month ticks over on the same date as the birthday. For a baby born on April 10, May 10 is one month old, October 10 is six months old, and April 10 the next year is the first birthday. That's why the half birthday is at six months.
- During the month of birth, the baby is "0 months old". The one-month checkup in health checkup notices refers to around the time the baby reaches one month.
- In a month without the same date, it's moved to the end of the month. For a baby born on January 31, one month old is the end of February (February 28 or 29), since February has no 31st.
- "X weeks old" advances by days. One week old is seven days after the birth, and explanations of the newborn period often count in weeks.
- Just like counting age in completed years, the baby is "X months" until the day before the date. Up to May 9 is 0 months, and from May 10 it's one month.
Procedures and checkups mix those set by days and those set by month age. Here are the typical ones in Japan.
| Procedure or checkup | When | Notes |
|---|---|---|
| Birth registration | Within 14 days, counting the birthday as day 1 | Check with your municipality how it works if day 14 is a holiday |
| One-month checkup | Around 1 month old | Follow the schedule given by the facility where you gave birth |
| Start of vaccinations | From 2 months old | Check types and doses in the maternal and child health handbook and municipal notices |
| 3–4-month checkup | At 3–4 months old | Your municipality sends a notice |
| 18-month checkup | At 1 year 6 months old | Timing varies somewhat by municipality |
The blood type field in the maternal and child health handbook is sometimes left blank. Many hospitals don't test newborns' blood type, and even if tested, the result can change before the body's antibodies have fully developed. Which type a child is likely to have based on the parents' blood types is explained in Blood type combinations and children's odds.
Working out the dates with the tool
Counting by hand, you'll trip over the different lengths of months. For large counts like day 100, it's more reliable to let a calculator do it.
- 1
Enter the date of birth
Enter the date of birth in the Find age tab of the Age & Anniversary Counter. It shows the age as "X years Y months Z days" plus the total number of days since birth. That display covers anything that needs the month age.
- 2
Look at the list of milestones
Milestones such as 100, 365, 500 and 1,000 days are listed with dates. It shows the three most recent past milestones and up to eight upcoming ones, so you can see the next one at a glance.
- 3
Convert to the customary count
The display counts the birthday as day 0. For ceremonies set by "day X after birth", such as Oshichiya and Okuizome, the correct day is the day before the date shown.
- 4
See the days remaining with the anniversary count
Once the date is set, enter it in the Anniversary count tab. It shows the days remaining, which helps you manage when to book photos or order food.
- 5
Share the URL with family
You can copy a URL containing what you entered. Send it to your partner or the grandparents and you can discuss the dates while looking at the same thing.
How to think about setting the dates
The day counts can be calculated exactly, but almost no ceremony has to happen on that exact day. Deciding your priorities in advance keeps the day itself calm.
- Many ceremonies don't need to be on the exact day. It's common to move Omiyamairi and Okuizome to a nearby weekend or a day when everyone is feeling well.
- Put the mother's recovery and the weather first. Going out in midsummer or midwinter is a strain, and shifting by a week or two is perfectly fine.
- Book the photos first. Studios fill up from the weekends, so it's more realistic to plan on separating the ceremony day and the photo day.
- Ask about guests' availability first. If grandparents are coming from far away, choosing a day that's easy to travel on rather than the exact day makes it easier to gather.
- Day-count milestones make good excuses for photos. After day 100 come day 500 and day 1,000, giving you chances to keep records at regular intervals.
Common points of confusion
| Point of confusion | How to handle it |
|---|---|
| Is the birthday day 0 or day 1? | Day 1 for ceremonies. Many elapsed-day calculators start from day 0 |
| Is one month old 30 days later? | It's the same date. Born April 10, one month old is May 10 |
| Month age for babies born at the end of a month | In a month without the same date, move it to the end of the month (January 31 → end of February) |
| If the baby was premature | There's chronological age counted from the birth date and corrected age counted from the due date. Development guidelines are sometimes judged by corrected age, so follow the facility's explanation |
| How it differs from kazoedoshi | Kazoedoshi, the traditional count, makes a baby 1 year old in the year of birth and adds a year every New Year's Day. It's a separate system from ceremony dates |
Few ceremonies have one absolutely correct way of counting, and there's variation by region and family. What matters is agreeing on one method within the family. The dates themselves can be calculated, so the only thing to decide is which custom to follow.
FAQ
- When is 100 days after birth? How do I count it?
- The birthday is counted as day 1, so day 100 is 99 days after the birth. For a baby born on April 10, 2026, it's July 18. Many elapsed-day tools count the birthday as day 0, in which case the "day 100" they show is one day later. When setting the date for Okuizome, look at the day before the one shown. Some regions hold it on day 110 or 120.
- How many days old is a baby at the half birthday?
- It's counted by month age, not days: the day the baby turns six months old. Month age advances on the same date as the birthday, so for a baby born on April 10, the half birthday is October 10. That's roughly 180 days, but months differ in length, so counting by days puts it a few days off. For babies born at the end of a month where the date doesn't exist, it's usually treated as the last day of that month.
- Is one month old 30 days after birth?
- Not 30 days later, but the same date as the birthday. For a baby born on April 10, May 10 is one month old. During the month of birth the baby is 0 months old, and the one-month checkup refers to around the time they reach one month. If the next month has no matching date, as for a baby born on January 31, it's treated as the end of February (February 28 or 29).
- When are Oshichiya and Omiyamairi held?
- Oshichiya is on day 7, six days after the birth. It's a ceremony where the baby's name is written on a naming certificate and announced, often held at home. Omiyamairi is said to be on day 31 for boys and day 32 for girls, though some regions use day 33 for girls. Neither has to be on the exact day, and it's common to move them to a nearby weekend to suit the mother's recovery, the weather and the guests.
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