Why it is not possible to schedule a Call on the weekend days?
When using the Schedule a Call button on the My Accounts page, Calls are automatically saved on the next Monday when they are scheduled for Saturday or Sunday.
This is expected functionality. Calls cannot be scheduled for non-business days with the Schedule a Call button. It is controlled by the locale of the user.
Example: if in an Arabic locale, save the Call on Sunday.
CRM Help Documentation: Accounts Overview