If your Zygopetalum is a complex hybrid and blooms a few times a year, it can be potted anytime of the year. I just hate potting them as the roots crack so easily that I have to be very, very careful. One of them got knocked over last month and was completely out of the rock and I had to completely pot it up again. It clearly less affected by the trauma than I was...it just kept growing and is now in bloom.