At a time: lúc / vào lúc, vào + day

For a clock time, "at" is lúc: lúc bảy giờ, at seven. For a day, a date, a month or a year, "on" or "in" is vào: vào thứ hai (on Monday), vào ngày mười (on the 10th), vào tháng tư (in April). Vào lúc is a more formal lúc, common in announcements and writing.

Where the time goes decides whether you need them. At the start of the sentence, they're usually left out: Bảy giờ tôi đi làm. At the end, they're usually kept: Tôi đi làm lúc bảy giờ.

To ask what time something happened, put lúc mấy giờ at the end: Anh tới lúc mấy giờ? What time did you get here?

Don't use ở before a clock time or a day.

Lúc can also take a whole sentence, like English "when": Lúc em tới, anh đang ăn cơm. When I arrived, you were eating. Lúc đó means "at that time", and lúc nào asks "when?". Khi is another "when", a little more formal on its own, though khi nào? (when?) is everyday speech.