yes it sounds like a script is needed, along with an OnObjectModify script trigger on that date object. The script would look like
If [ DayOfWeek ( TO::DateField ) <> 1 ]
Show Custom Dialog - "not a Sunday."
Go To Field, Select, TO::DateField
awesome, thanks David!
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It's not the only option.
DayOfWeek ( Self ) = 1
Could be added as a calculated Validation field option and then an error is shown if the date entered is not a sunday.
My preference though would be to craft a custom "date picker" that only allows the selection of Sunday dates in the first place. Better to prevent incorrect data entry than to allow it and then trap for the error after the fact--forcing the user to try again.