Server restart is around 5:00-5:30 a.m. Serbian time (which is I believe GMT+2). At that time for about 20 mins you can' log in. During this time they update everything, promote you to higher level and do all the other stuff. Any other time you play the game normally except like an hour before the restart the transfermarket clears out. Use the time between your last game and the server restart to play some friendlies! And I don't think you'll sell anyone on the last day (especially if you're on low level), better sell at the beginning of new season.