noun, used as a suffix
day of the month
Attaches to numerals to express a specific day of the month, e.g. 一日 (ついたち), 二日 (ふつか), 三日 (みっか). Note the irregular readings for some days.
今日は何日ですか。
What day of the month is it today?
会議は三日に延期された。
The meeting was postponed to the 3rd.
counter
Used after a number to count days, e.g. 一日 (いちにち), 二日 (ふつか), 三日 (みっか). Note the irregular readings for small numbers.
旅行は三日間でした。
The trip was for three days.
あと二日で夏休みが終わる。
Summer vacation ends in two more days.
Also means 'day' but is used in different contexts, such as days of the week (e.g. 日曜日) or as a counter for days in formal/legal contexts (e.g. 第１日).
Means 'day' as a standalone noun, often referring to a day in general or a specific date, e.g. その日 (that day).
The reading か is the native Japanese (kun'yomi) for the kanji 日, used specifically in counting days and dates. The irregular readings for the first ten days (ついたち, ふつか, etc.) are remnants of Old Japanese.