WHERE CONVERT(varchar(2), Month(ADM_Date), 126) in ( @Month )
DECLARE @sql VARCHAR(MAX)
-- DECLARE @Month VARCHAR(50) SET @Month = '1,2,3'
SET @sql = 'SELECT ... ' -- Your Query
SET @sql = @sql + ' WHERE Month(ADM_Date) in ( ' + @Month + ' )'
EXEC (@sql)