一日: a date or a duration?
Learn both ついたち and いちにち for 一日. Distinguishing a calendar date from a length of time helps you interpret the sentence.
The first of the month: ついたち
For the first day of a month, 一日 is normally ついたち. A month name or a scheduled event can help identify this use.
四月一日に始まります。 means “It starts on April 1.” Here 一日 is ついたち, not a duration of one day.
A day: いちにち
Use いちにち for a duration or unit of one day. 一日かかります。 means “It takes a day”; 一日に三回 means “three times a day.”
The particle に alone does not determine the reading. It can follow a calendar date or a unit used to express frequency.
Recall the reading and meaning together
A card that says only “一日 = a day” can hide the date reading. Practice “May 1” and “takes a day” as separate contexts.
After reading aloud, say whether the phrase means a date or a duration. A correct pronunciation with the wrong interpretation still needs practice.
Quick check: How do you read 一日 in 五月一日 and 一日かかる?
Show the answer and why
五月一日 is ごがつついたち; 一日かかる is いちにちかかる. The first is May 1, and the second means to take a day.