The new snapd 2.25 has a new feature to set the refresh schedule within a 24h window. This allows to say that you want updates only between 4:00-7:00 in the morning for example or between 4:00-7:00 and 19:00-22:00.
The syntax is:
$ sudo snap set core refresh.schedule=4:00-7:00/19:00-21:00
Note that a refresh may happen outside of this window if your last refresh is too old, in this case snapd will refresh even outside the scheduled windows.