There is no SH that departs on xmas day. However there are some long haul. Last year there were 3 LH flights that departed on xmas day.
There are a few probabilities:
1. You will arrive back from somewhere on the 24th. (Short Haul)
2. You will arrive back from somewhere on the 25th. (Long haul)
3. You will go somewhere on the 24th and land back on the 26th. (Short haul)
4. Depart on the 24th, 25th or 26th for a trip anywhere from 3 days - 7 days.
There is a bidding pack that all the crew get whereby you can choose which flights you'd like to over xmas and new year. However it is only a preference and by no means mean that you will actually get it.
In short you can tell them what you would prefer but you get what your given and it can be anything.
Hope this helps a little and welcome to LGW
