for once

(just) for once and just this once {infml} on this occasion (which is in contrast to what happens usually) — в кои-то веки; хотя бы раз (в жизни), хоть раз (в жизни), хоть когда-нибудь, (хоть) изредка, (хоть) иногда

Example 1: For once I wish you’d tell me the truth.— Хоть раз в жищни можг бы сказать правду. В кои-то веки мог бы и правду казать.

Example 2: Just for once she arrived on time.— В кои-то веки она вовремя приехала.

Example 3: You can pay the bill for once.

Example 4: It would be nice if, just for once, the two of you could get on with each other. Can't you be nice to each other just this once?

Used for saying that you would like something to happen on this occasion, even though it does not usually happen, and you think it should happen more often

Сf:
(just) the once
(=on one particular occasion only)— только (один) раз; всего лишь (один) раз; (один) единственный раз

Example: We met just the once, but I still remember him very clearly.

Related vocabulary:
[в кои-то веки]
[once in a blue moon]