四月一日 is April 1st. Read it the way your first month of Japanese taught you, one number and one 日 at a time, and you get shigatsu ichinichi, which sounds like "one day in April." The date is shigatsu tsuitachi. Nothing in 一 or 日 hints at tsuitachi, which is why most guides to Japanese days of the month hand you a 31-row table and tell you to memorise it.
The table hides three facts. The 2nd to the 10th are the hitotsu, futatsu counting numbers (if you haven't learned those yet, learn them alongside this, because the dates come almost free with them). The 1st is a leftover from the lunar calendar. And the 20th borrows an old word for "twenty" that still lives in hatachi, the word for twenty years old. Learn those, and the table shrinks to about a dozen forms, most of which you can rebuild from scratch.
Why the 2nd to the 10th are the numbers you already know
ふたつ (futatsu) is "two things." ふつか (futsuka) is "the 2nd." Strip the つ off the native counting word, add か, and you're most of the way there for every date up to ten.
Keep the native number, swap つ for か.
The rest of the set, native root on the left:
- ふた → ふつか (futsuka), the 2nd
- よ → よっか (yokka), the 4th
- む → むいか (muika), the 6th
- なな → なのか (nanoka), the 7th
- や → ようか (yōka), the 8th
- とお → とおか (tōka), the 10th
That か is an old native counter for days. Wikipedia's Japanese calendar entry puts the whole rule in one line: days use native readings up to ten, Chinese-derived readings after.
A few roots bend on the way. Wiktionary calls the ふつ in ふつか an irregular reading, apparently shifted from ふた. みっか grew its small っ from an older mika. ようか goes back to yauka, usually traced to an earlier yaka: や plus か.
I couldn't find a dictionary that explains the い in むいか, so treat it as the one piece of pure memorisation. The 7th also has an older form, なぬか (nanuka), which is where なのか came from.
The same words count days, too. ふつか is "the 2nd" and "two days." A hangover is 二日酔い (futsuka-yoi), "two-day drunkenness," and someone who drops every new hobby after three days is a 三日坊主 (mikka bōzu): a three-day monk. Add 間 (kan) for an explicit stretch of time, as in 七日間 (nanoka-kan), "seven days."
Where tsuitachi comes from: the lunar calendar's month start
ついたち is a worn-down つきたち (tsuki-tachi), 月立ち: the month starting, and since 月 means both month and moon, the moon too. Kotobank's encyclopedia entry give it as a sound change of つきたち. It's also written 朔日, with the character for the new moon.
That made literal sense for most of Japanese history. Japan ran on a lunisolar calendar until the Meiji government switched to the Gregorian one, and the National Diet Library records how abruptly: what would have been the 3rd of the 12th month of Meiji 5 (1872) became January 1st, 1873.
The lunar calendar named the month's end as well. The last day was tsugomori, "moon hidden." The word you'll actually meet is みそか (misoka), from an old native みそ, "thirty." That's why New Year's Eve is 大晦日 (ōmisoka). The big last day.
Tsuitachi or ichinichi: when 一日 means a date and when it means a day
一日中 (ichinichi-jū) means "all day long." Same two kanji as the 1st, different word. Read いちにち, 一日 is a length of time. Read ついたち, it's a spot on the calendar.
A date takes ついたち. いちにち is a duration: 一日に三回, three times a day.
It works the other way round as well. The sci.lang.japan FAQ notes that the other date names can take 間 to make an explicit duration, but ついたち never does. "One day" is always いちにち.
Why the 20th is hatsuka (and what it shares with hatachi)
二十日 is はつか (hatsuka), not nijū-nichi. The clue is はたち (hatachi), "20 years old." Both share はた, an old native word for twenty.
In The Tale of Genji, a character counts on her fingers "十、はた、三十、四十": ten, twenty, thirty, forty, as quoted in Daijisen. Daijisen adds that in modern Japanese はた normally appears only with ち attached, as はたち.
How はた turned into the はつ of はつか is less settled. Wiktionary offers two competing sound-change stories and flags both as unverified. I won't pick one. The shared root is what you need to remember it, and it turns up elsewhere: the little red radish is 二十日大根 (hatsuka daikon), a name usually explained by its harvest, as little as twenty days after planting.
If you've read about why one pen is ippon, はたち was the odd one out in the age counter. Same はた.
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Try Conversa FreeWhich Japanese days of the month are irregular? Only the 14th, 20th and 24th
十一日 is jūichi-nichi. Past the 10th, Japanese switches to the Sino-Japanese numbers you use for prices and phone numbers, plus にち: 十五日 jūgo-nichi, 三十一日 sanjūichi-nichi. Three dates break the pattern:
- 十四日 じゅうよっか (jū-yokka), the 14th
- 二十日 はつか (hatsuka), the 20th
- 二十四日 にじゅうよっか (nijū-yokka), the 24th
The 14th and 24th are じゅう and にじゅう bolted onto the よっか you learned for the 4th. So the "irregulars" after the 10th come down to one word you've met twice, plus はつか.
Some date tables also colour the 19th and 29th as irregular. They're a different kind of irregular: a choice between two readings of nine. sljfaq calls じゅうくにち the more common reading for the 19th, and Wikipedia lists じゅうきゅうにち as well. The 17th swings between じゅうしちにち and じゅうななにち.
Yokka or yōka: the four-day mix-up natives make too
よっか (yokka, the 4th) has a short よ and a small っ. ようか (yōka, the 8th) has a long よう and no っ. Pitch won't rescue you. Dictionaries list both as flat, so the only difference is where the length sits: on the consonant in よっか, on the vowel in ようか. It's the same beat-counting skill that separates おばさん from おばあさん, except here a missed beat moves your appointment by four days.
You meant the 4th: short よ, then a held っ. ようか is the 8th.
sljfaq's mnemonic: eight days is longer than four, so ようか gets the long vowel.
The mix-up is old. According to Wiktionary's entry for 四日, the earliest kana spellings of "the 4th" were ようか, and the shift to yoka and then よっか may have happened to stop it colliding with 八日. If so, the language patched its own bug. Natives still trip over the neighbouring dates, though: in a Rocketnews24 column, a 37-year-old Japanese writer admits he still can't instantly tell whether はつか is the 8th or the 20th, because it starts with the は of はち.
On the phone, some people dodge the problem by saying はちにち. A customer-service trainer goes further and has staff confirm with the weekday: ようか、金曜日でございますね, "the 8th, a Friday, correct?"
Steal that. Booking anything for the 4th, 8th, 14th, 20th or 24th? Say the weekday too.
How to ask and say the date in Japanese
今日は何日ですか (kyō wa nan-nichi desu ka) asks "What's the date today?" The answer runs month first, then day: 今日は四月八日です, kyō wa shigatsu yōka desu, "Today is April 8th."
Festival days give you free practice, because several fall on matching month and day numbers:
- ひな祭り, Girls' Day: 三月三日, sangatsu mikka
- こどもの日, Children's Day: 五月五日, gogatsu itsuka
- 七夕, Tanabata: 七月七日, shichigatsu nanoka
Each one pairs a Sino-Japanese month with a native-number day. Say all three out loud and you've drilled the switch between the two systems in one breath.
Nine native-root dates. ついたち, with its moon. はつか, with its はたち. Everything else is a number you know plus にち, with よっか reused twice.
Start with your own birthday and say it in full. If it lands on the 4th, 8th, 14th, 20th or 24th, that's your drill for the week, and the weekday trick covers you until it sticks.