so it would depend on how you want to do it. For example, Weekday(DatePicker1.SelectedDate) will get you the numerical day of the week for a selected date
So if I feed it December 25, 2020, it gives me 6.
So if you want the next day of the week after a holiday, you could set the default date of another date field to
DatePicker1.SelectedDate+1
If by week date you mean next working date, you would have to define what your workplace rules are.
For example, could be something like this if you say that if a holiday happens on on Friday-Sunday, the next weekday should be Monday:
If(
Weekday(DatePicker1.SelectedDate) < 8,
DatePicker1.SelectedDate + 1,
If(
Weekday(DatePicker1.SelectedDate) > 5,
DatePicker1.SelectedDate + 2
)
)