Hello Devlend,
I think that you keep getting "Other" because the dates in Clarizen include the timestamp as well as date (i.e. Due Date is probably not 0 hours and Date generated by Date() is 0 hours).
I'm not sure if this will work in report formula column, but you could try something like this: Case(Date(Year($DueDate), Month($DueDate), Day($DueDate)), Date(2019,12,13),'Sprint 1', Date(2019,12,17),'Sprint 2','Other')
I hope this helps.
Roland