The deadline for the initial application round (to get 'equal consideration' from Universities) is 6pm on 15 January.
The UCAS website usually gets swamped with applications on this last day. This can mean it freezes or even crashes. There is nothing more stressful than an a teenager desperately trying to submit their application on this last day and not being certain if its 'gone' or not. And watching 6pm approaching .......
SO DONT LEAVE IT TO THE LAST MINUTE. If they submit their application a few days before this deadline, they will avoid all this panic/worry.
[And remember, no teacher is going to want to write a last minute reference over Christmas, so make sure your DC gets it all sorted this week - for your own sanity.]