Yes, I meant the full scan that Avast has created like DavidR said above. The one on the left when you open the scans' tab. And it was possible to change its start time not too long ago, it was messed up with one of the last couple of updates.
I attach a screenshot with my current scans as well, the default full scan is the one called 'Käivita täielik viiruste kontroll' (start full virus check in Estonian) and as you can see it's currently scheduled to start on Saturday (laupäev) at 22:12, that's the time I have scheduled it for when it was still possible but now when I open its settings using the 3 dots next to it there is no way to re-schedule it and I can't disable it either, so it starts on Saturdays at 22:12 no matter what, which is annoying as I would like to have control over if and when to run it!
And actually there is another bug! It's related to the scan under my scans called 'Kiirkontroll' (fast scan). As you can see it is currently scheduled to run on Mondays (E), Wednesdays (K) and Fridays (R) at 21:53. Now the interesting part: this scan does actually have scheduling in its settings (weird as full scan doesn't) but it is now actually not possible to pick those 3 days and only options are never, once, every day, once a week and once a month. Every day setting is displayed as chosen in the settings (I haven't chosen it btw) but actually it still runs on Monday, Wednesday and Friday as I have specified in the past! What a mess