Fun With Dates in Excel

This will always give the date of the next Sunday:

=A1 + 7 - WEEKDAY(A1)+1

If A1 is a Sunday, this will give the following Sunday (e.g. if A1 is Nov. 18, it will give Nov. 25).

If instead, when A1 is a Sunday you want it to give you the same date, rather than the following Sunday (e.g. on Nov. 18, it gives Nov. 18), then use this version:

=$A1 + 7 - WEEKDAY($A1,2)

To find the first day of the following month, use the following EOMONTH function:

=EOMONTH(TODAY(),0)+1