Don\'t do it.
While it is possible to postpone it, you may not get an early date. Reserve postponement for "emergencies". Workload is not an emergency. Moreover, how can you be sure that the workload issue won\'t come up the next time?
Impress upon your manager the importance of this matter and try to negotiate (unpaid leave, extra hours before leave, etc.)
At the risk of sounding patronizing, I will say that superiors will bend you as far as you can bend. Occasionally you need to push back.
Good luck.