I am building a process model that contains a start date and an end date picker.

i don't know if there is a specific function but a workaround i can suggest:

1- Create a first custom system calendar "startMonday" where you only select all monday as working days and all the rest as Non working days.

2- Create a second custom system calendar "endSunday" where you only select all sunday as working days and all the rest as Non working days

3- For each dates use the related calendar with the Function : caladddays() passing 0 as offset.

i mean for startDate you would use: caladddays(startDate,0,"startMonday")
for endDate you would use: caladddays(endDate,0,"endSunday")

i think this should work and you can modify calendar to include/exclude more Days

To validate if the date picked is a monday or not you could use a custom expression rule that checks to see if the appian function weekday(dateSelected)=2 (Monday is the second day of the week).

You can do something similar for sunday (weekday() = 1) but if its going to just be one week you can update the end date to startDate + 6.

Thank you both! I will give your suggestions a try.