"Kiedy odpoczywaliście?"

Translation: When were you resting?

[Kate821907]

when have you rested?

[Jellei]

Well... it's often hard to decide which past tenses should be accepted for a perfective/imperfective, but it seems possible... especially that 'when did you rest' is accepted right now. So added.

[Erik787431]

When were you relaxing?

[Jellei]

No. There is a different verb for relaxing. "relaksować się".