Well, I would think that the orbital period is 1 year for Calypsians, but, in relation to earths orbital period, which we should be using to measure other orbital periods, it is 1yr and 91 days.
Actually its dog years so expect the next migration in 2014...
Seriously, I would really like to see this sort of migration happen on a regular basis. It should happen every year at the same time, just like in real life.