Aquamail has "Nights and Weekends" settings (General app settings, as well as per-account settings)
AFAIR, "Weekends" portion allows only to switch the IMAP off, not the other way around.
The workaround for that would be to choose your weekends M-F, and your weekdays Sat-Sun.
Then you enable IMAP for the new "workdays" (your Sat-Sun), and checkmark "Disable IMAP Push" under "weekends" (your M-F).
And you have the scheduled sync at all time - this is actually recommended even when you have a Push. But you can change the frequency of it during the new "Weekends", if you insist.