What Elvia said.
I personally run 400iu twice a week, and also think that 250iu on Fri and Sat is good and actually better than 500iu in one day, but not so much if it was inconvient to do so.
There was a study done that showed that 250iu eod was not quite enough and that 500iu was a little too much, doing the math 400iu eod was perfect in the study, but I really don't think that it took into account the frequency aspect so much as 400iu twice a week works perfect for me and 250iu twice a week on Consecutive days (i.e fri,sat) worked fine too.