mmm, that’s an interesting one.
The Date Picker component does provide an option to only allow dates in the past, but not specific days of the week. However, that doesn’t mean it isn’t possible.
How far back would you want to go? You mentioned the past 20 Mondays - is that it, or could it be any Monday in the past? 1 year ago, 20 years ago?
It wouldn’t be too difficult to dynamically generate the values for a select list based on the current date, using a bit of date math. But that probably wouldn’t be a practical option if there was no limit to how far you could go back.