If you want a broadcast (text blast) to go out every week or every month, you have 2 options:

1) Use the Send Blast section of your Messaging menu to schedule your blasts individually for the dates and times you need them to be sent. 

See Related Article: Messaging > Send Blast

2) Set-up a Trigger Campaign to automate the daily, weekly, and monthly delivery of your messages. 

See Related Article: Messaging > Trigger Messaging