It is better to freeze cells when they are in the growth phase or when cells are growing rapidly. You can plate the adherent cells in a petridish or in T 75 tissue culture flask and add fresh medium the day before you freeze the cells. Cells should be close to confluent, covering most of the surface. At this stage they are growing rapidly and with fresh supply of medium, they are happy and healthy. If you are freezing suspension cells make sure they are also in the growth phase before you freeze them.