I hate to beat this dead horse, but the corrected question/answer went out today.
Explanation: Either of the first two answers will work. All the month names have at least the letter 'a' once. The month function returns the number of the month, that can be compared against the month number of given months
In Spanish, the month of february is febrero, in french it's Février, etc... and depending on the language settings your SQL server is set to, you'll get different results.
Because of that, datename(m,join_date) like '%a%' is not 100% valid.