Is it preferable to run an in-ground pool cleaner through the night or the day?  Or does it matter?

Most local power companies request that pool equipment be used after peak hours aka night time. This is to help spread the demand load out so that the grid can handle the peak loads. A pool cleaning robot can do a more efficient job if it is operated when the pool in not in use stirring up the water and the debris. So the win-win answer is to run your system at night!

Depends on a few factors. Does your electricity providor have peak times and off peak times for billing? Then off peak is night time.

night I guess so u can use it in the day