Generally you don't have to clear out at Christmas. At Easter, it depends what room you are in and what the College's conference plans are. Colleges subsidise their operations (ie students) by taking conferences during the vacation periods. So if you are in an old, central room and it's en-suite, they can make £100-£150 a night selling that to a conference. If you are in a small room, with a shared bathroom, in a Victorian terrace, down by the railway station, they can't let that out so you can probably leave your stuff there year round.