can you scedule Avast to scan at a set time /day ?
Hello and welcome to the avast! user forums,
Yes and No. If you are using the free Home Edition of avast! this feature is not available. If you are using the paid Professional Edition version then yes you can.
Welcome to the forums, remember to visit again even if your not having problems, maybe you can help somebody, or learn something new. 
~justin1278
Check This out -
HOWTO: Scheduled Automatic Scans avast! Home Version!
The major disadvantage with the task scheduler trick of using ashquick.exe is, it will scan every file of the hdd, partition, folder or file that you set it to scan; even those files not considered a potential threat, this can take a long time.
It is possible to add certain folders to the exclusions list for say your media files, tec. This could leave you vulnerable if that folder got infected as it wouldn’t be scanned by the on-demand scanner as you have excluded it.
The Pro version gives a lot more flexibility and the scan would follow the settings and exclusions, etc. of the regular scan.
With the different levels of protection of protection and the fall back of the Standard Shield, I find the need for daily scheduled scans lessened. I prefer to do a manual scan with my regular weekly maintenance tasks.
If you have the Pro version this is how you can manage this:
Right click the tray icon\Start avast! Antivirus\Choose Tasks in the left pane\Pick or create the task you want to perform\Right click it and select Properties\At the bottom of the left pane click Sheduling\Add\Set up a shedule.