I want to sent multiple reports (group membership lists) to different individuals (the co-ordinator of each group) on the 1st day of each month. Scheduled jobs only allows hourly or daily mailing (not monthly) so I believe I may have to use CiviRules, but can I trigger a rule for a specific day of the month? We use Civi with Drupal.
Ramesh's comment is correct. The best overall solution is most likely to upgrade to CiviCRM 4.7, which adds a "Monthly" option.
If upgrading is not an option and you're on CiviCRM 4.6, you can install the Flexible Jobs extension to get this functionality.