we need choise weekend days here in middle east the weekend is friday and satuday.
Stay in Mendix as long as you can, easier for other devs in the team.
For complex date time calculation you can divert to java with Joda Time
No need for Java for this particular problem!
You can use the
parseInteger(formatDateTime($InputDateTime, 'u')) = 6
expression to determine the day of a given dateTime, where 1 = Monday, 2 = Tuesday, etc.
So in your case that would be
parseInteger(formatDateTime($InputDateTime, 'u')) = 5
or parseInteger(formatDateTime($InputDateTime, 'u')) = 6
where 5 is Friday and 6 is Saturday.
Als see the Mendix Docs on date functions