Do you keep the same time/day to facilitate the retro?

I completely agree with Enrico about trying to keep it consistent and on the same day to minimize disruption. 

Whatever you do, it is up to the SM or Agile Coach to teach the team and management how important the retrospective meeting is. Without the proper appreciation for the meeting it will move from regular, to sporadic, to email based because of "urgent" work. If you don't protect the retro, then more and more work will become urgent. 

Respecting the meeting by keeping it regular no matter what else is happening is a good start to valuing the practice and getting much more out of the time invested. 

If it's an iteration retrospective yes I schedule it at a consistent day/time. Themed retrospectives are scheduled as the team see fit at a time/day that works for the audience. 

Somewhat related is that I try to schedule iteration planning, demos and retrospective on the same day to minimize disruption on the team.