noun
2nd day of the month
Refers specifically to the second day of a calendar month. Often used with month names or in date expressions.
会議は来月の二日です。
The meeting is on the 2nd of next month.
noun
two days
Indicates a duration of two days. Often used with counters or in expressions like 二日間.
This work will take two days.
一日 means the 1st day of the month, while 二日 is the 2nd day. Both are irregular readings for date expressions.
二日間 explicitly marks a duration of two days, while 二日 alone can mean either the date or the duration depending on context.
The reading ふつか is an irregular native Japanese counting form for 'two days' or 'the second day', not derived from the regular numeral 二 (に). The exact historical derivation is uncertain.