Hi MikeB,
For Classic - The option you mentioned is likely the fastest and cleanest way to make this happen. You'll also need to add some actions on the Add/Edit screen of whatever records you have in your grid to populate information onto your "Month" dropdown field when the record is actually created to make things simpler.
In Method:New - we have added functionality for a DateTime Comparisons to Compare using only particular parts of the DateTime field (e.g only compare the Month), but this functionality does not exist in Classic.
Thanks,
-Ben