Are they bid for like leave weeks, or are they company allocated?
They're bid for although it's based upon a points system rather than seniority. In essence, you get more points for unpopular weeks (middle of October) than popular weeks (Christmas, middle of August, etc). The more points you have (based on the previous eight seasons), the more choice you have - so bid for unpopular weeks initially to build up your points to give yourself more choice later on. Leave is allocated the same way.