Your best bet would probably be to go with the rules module (https://drupal.org/project/rules)
Create a new component in rules that checks if a user account was created one month ago, if so, send an email. Then schedule that rule to run every day using the scheduler.
You'll find many helpful tutorials here: https://drupal.org/node/1866108
Probably 1-6, 14, and 16 would help you a lot.
EDIT: As suggested in a comment, it is bad for performance to check every user account every day when you have a large user base, rather schedule directly on user registration.