Counting and Naming Days (日)

If you learn only one irregular counter in Japanese thoroughly, make it 〜日（にち）— the counter for days of the month and for stretches of days. Its first ten values don't follow the neat ichi-ni-san pattern at all; they are built on the old native number roots you met in ひとつ・ふたつ, and they must be memorized as a fixed set. There is no shortcut and no clean rule to derive them — this is, without exaggeration, the single most irregular counter in the language. The good news: it's a closed list of about a dozen forms, and once they're in your head they never change again.

The native-root block: days 1–10

Here is the block that trips up every beginner. Days one through ten of the month use readings drawn from the ancient native counting roots (ふた-, み-, よ-…), not the Sino numbers.

Why they look like the native numbers

Nine of these ten are not random. Line them up against the native series ひとつ〜とお and the family resemblance is obvious: ふつか ↔ ふたつ (2), みっか ↔ みっつ (3), よっか ↔ よっつ (4), いつか ↔ いつつ (5), むいか ↔ むっつ (6), なのか ↔ ななつ (7), ようか ↔ やっつ (8), ここのか ↔ ここのつ (9), とおか ↔ とお (10). The 〜か on the end is an old word for "day." So really you're re-using a number series you've already met — see Native Numbers: ひとつ〜とお.

The one true oddball is the 1st. ついたち has nothing to do with いち. It comes from 月立ち (つきたち → ついたち), literally "the moon rising" — the day the new moon begins the month. This is why you can never say いちにち for a date.

Days 11 onward: back to regular 〜日

From the 11th, the counter finally behaves. You take the Sino number and simply add 〜にち — with three important holdouts.

Three dates in this range refuse to be regular:

• 14日 じゅうよっか and 24日 にじゅうよっか — the "4" keeps the native よっか from the block above, so it is jū-yokka, never jū-yon-nichi.
• 20日 はつか — a completely irregular native form, related to the old word はた ("twenty," the same root inside 二十歳 はたち). It is never ni-jū-nichi.

Everything else — the 17th jū-shichi-nichi, the 19th jū-ku-nichi, the 30th san-jū-nichi — is just Sino number + にち. Note that 17 uses しち (shichi) and 19 uses く (ku) for the "day" readings, matching how those numbers behave in dates and times generally.

One reading, two meanings: date and duration

Here is a feature English speakers rarely expect: for the 2nd through the 10th (and the 20th), the same word names the date and counts the span of days. 二日 is both "the 2nd (of the month)" and "two days"; 三日 is "the 3rd" and "three days"; 二十日 is "the 20th" and "twenty days." Context tells them apart.

The lone exception is day one. As a date it is ついたち; as a duration ("one day") it is the regular Sino reading いちにち ichinichi. This split is the heart of the classic beginner error — see the mistakes below.

When you want to make "for N days" unambiguous, you can add 間（かん）: 三日間 mikkakan ("for three days"), 十日間 tōkakan ("for ten days"). The 間 pins the meaning to duration and is common in schedules and travel talk.

Dates take に; the count of days does not

When a 〜日 form is a calendar date, it behaves like a specific point in time and takes the particle に: 二日に会おう ("let's meet on the 2nd"). When it's a duration, it does not — durations attach directly to the verb. This is the same に you use for 七時に and 月曜日に; see に: Specific Points in Time.

Common mistakes

The number-one error is regularizing the 1st to いちにち. That form exists, but it means "one day" (a duration) — never the date. The 1st of the month is always ついたち.

Ni-jū-nichi is the trap everyone falls into once they've learned that 11–19 are regular. The 20th breaks the pattern: it is はつか, a fossil of the old word for "twenty."

Because 4 is usually よん elsewhere, learners produce yon-nichi. But the 4th, 14th, and 24th all keep the native よっか: yokka, jū-yokka, ni-jū-yokka.

A subtle one: ようか (8th) and よっか (4th) sound close but are different — ようか has a long ō and no small っ; よっか has the held っ and a short vowel. Mixing them up will move an appointment by four days, so drill the pair until the contrast is automatic.

Key takeaways

• Days 1–10 use native-root readings: ついたち, ふつか, みっか, よっか, いつか, むいか, なのか, ようか, ここのか, とおか — memorize them as a set.
• ついたち (1st) is unrelated to ichi; it means "the month begins." The others echo ひとつ・ふたつ.
• From the 11th it's regular Sino + にち, with three holdouts: 14日 じゅうよっか, 24日 にじゅうよっか, 20日 はつか.
• For the 2nd–10th and 20th, the same word is both the date and the day-count (二日 = "the 2nd" or "two days"); only day 1 splits into ついたち (date) vs いちにち (duration).
• A date takes に (四日に); a duration does not, and can add 間 for clarity (三日間).