Back when: hồi + time (hồi nhỏ, hồi đó)

Hồi points back to a time in the past, like "back when". Put it before an age, a time or a whole sentence, usually at the start: Hồi nhỏ em ở Cần Thơ. When I was little, I lived in Cần Thơ (in Hà Nội, hồi bé). Hồi còn đi học, em hay đi xe đạp. Back when I was at school, I used to ride a bike.

Hồi đó means "back then" (in Hà Nội, hồi ấy), hồi trước "before, in the past", and hồi xưa "long ago, in the old days". Before a time word, hồi simply places it in the past: hồi tháng trước (last month), hồi năm ngoái (last year). In Saigon, hồi sáng is "earlier this morning" and hồi nãy is "a moment ago".

Because hồi already says "in the past", you don't need đã.

Khi and lúc also mean "when", for any time, past or future. Hồi is only for the past, and in Saigon it's the everyday word for it. To ask "when?" about the past, put hồi nào at the end: Anh về hồi nào? When did you get back?