How mysql generates monthly events?

How mysql generates monthly events?

I want to create a monthly event for every registered customer that fires
an insert statement on billing table. If an event is specified to run from
say 20th Aug 2013 0:00 hrs (a future date), on what date can I expect next
event to occur?
I don't know whether to assume it as 30 days or 365/12 days (mathematically).
I am using something like this to create that event:
DELIMITER $$
CREATE EVENT event1
ON SCHEDULE EVERY '1' MONTH
STARTS '2013-08-20 00:00:00'
DO
BEGIN
END$$
DELIMITER ;
My question is when does mysql fire this event in the next month?