This comes to mind:
https://rtcamp.com/tutorials/wordpress/wp-cron-crontab/
and
http://www.smashingmagazine.com/2013/10/schedule-events-using-wordpress-cron/

Create cron to:
– Auto-delete older unpayed bookings (1 param: age of booking, runs 1ce per hour)
– Send a reminder x days before an event starts (3 params: time before event, mail subject template, mail content template)
– Send a reminder to unpayed registrants x days before an event starts (3 params: time before event, mail subject template, mail content template)