何なん日にち

な

ん

に

ち

Meanings

what day; which day

Asks for a specific day of the month or day of the week. Often used with か or in questions.

how many days

Asks about a duration or number of days. Often used with counters or in expressions like 何日間.

今日きょうは何日なんにちですかwhat is today's date
今日きょうは何なん日にちですか。
What's the date today?
〈事こと〉に何なん日にちかかるhow many days it takes to do something
東とう京きょうから大おお阪さかまで歩あるくと何なん日にちかかりますか。
How many days does it take to walk from Tokyo to Osaka?
何なん日にち間かん〈動どう詞し〉for how many days (doing something)
何なん日にち間かん旅りょ行こうに行いく予よ定ていですか。
How many days are you planning to travel for?
〈時とき〉から何なん日にちhow many days since/from a certain time
生うまれてから何なん日にちたちましたか。
How many days has it been since he was born?

幾いく日にちN1
幾日 is a more formal or literary way to ask 'what day' or 'how many days', while 何日 is the everyday word used in speech.

何なん曜よう日びN5sense 1
何曜日 asks specifically for the day of the week (Monday, Tuesday, etc.), while 何日 asks for the date of the month or a number of days.
何なん月がつN5sense 1
何月 asks for the month of the year, while 何日 asks for the day within a month or the number of days.
何なん時じ間かんN5
何時間 asks how many hours something takes, whereas 何日 asks how many days (or which day of the month), so they're easy to mix up when asking about time spans.

日にちN3sense 2
何日 asks 'how many days', and the answer is given with the counter 日（にち）, as in 三日 (three days).
日か sense 2
For durations of days, 何日 is often answered with the counter 日（か）, as in 五日 (five days), used for small numbers.

Compound of 何 (nan, 'what') and 日 (nichi, 'day'). The reading なんにち is a straightforward combination of the two elements.