>>Use TinyInt for this.
>>
>>https://docs.microsoft.com/en-us/sql/t-sql/data-types/int-bigint-smallint-and-tinyint-transact-sql
>
>What advantage (other than smaller storage) the tinyint would have vs. NUMERIC(2,0)?

In SQL Server (as well as in other languages) you want to use the correct data type for the columns. In this particular case having tinyint for the months will be self-explanatory for the purpose of these columns. And, of course, the smaller storage is also an advantage.