〜までに
by (a deadline)
Time / Verb dictionary form + までに
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までに sets a deadline: 金曜日までにレポートを出してください, "please hand in the report by Friday". The action happens once, at some point before the limit.
Compare まで, "until", where the action continues the whole time: 金曜日まで休みます, "I'm off until Friday".
A quick test: if English says "by", it's までに; if it says "until" or "till", it's まで.
It works after verbs too: 夏休みが終わるまでに, "before the summer holidays end". And 何時までに asks for a deadline: 何時までに来ればいいですか, "by what time should I come?" Shop signs and forms use it all the time.
Examples
金曜日までにレポートを出してください。
Please hand in the report by Friday.
五時までに帰ります。
I'll be back by five.
夏休みが終わるまでに、この本を読みたい。
I want to read this book before the summer holidays end.
明日の朝までに返事をください。
Please reply by tomorrow morning.
何時までに来ればいいですか。
By what time should I come?