You can schedule on a non-work day (weekend or holiday) by clicking on the day and confirming.
Tasks scheduled on a non-work day will appear over a red background with red diagonal lines on the Schedule. They are recorded as overtime hours in your reporting.
💡 Here's a tip: If you've set your account to hide non-work days in your Team settings, you will not be able to schedule on weekends. Unhide the weekends to resume scheduling on these days.