Used to describe something that you anticipate will be belated (e.g. wishing someone happy birthday) had it not occurred at this instant (now), at a time which is earlier than "on time".
Most commonly seen on Facebook walls where one friend wishes another a happy "prelated birthday" (in case they forget to do so on the day of).
Wishing someone a happy Birthday before the date of their Birthday because you know you will forget on the day to wish them or will feel bad to wish them after.