Yep, it is.Think it's at the end of each transfer window
Whatever 25 we name on 1st of September cannot be altered until January, and then whatever we name at the end of January will do us for the rest of the season.
The only exception outside the transfer windows is if a player in the 25 suffers a long-term injury (I think that is defined as one which will keep them out for at least 2 months) in which case we can ask permission to replace them in the squad.