It depends on the university. Most universities have reverted their interviews back to face-to-face post-covid so won't offer online interviews. You may be able to email them when you receive your interview to see if they will change it or have any slots free on another day. Some universities like Sheffield will allow you to book your slot as and when it suits you while others will give you a slot and will not be able to guarantee a change in date. It also depends on your UCAT score as some universities look at when to offer you an interview from the top scorers to the bottom up until their threshold for that year of entry. If you are mid-way down the list then perhaps you will be invited later. Most universities do close for Christmas break though so you should be alright - fingers crossed. But universities vary in how long they close for. You'd have to also prepare for interviews before your holiday so you are prepared if you get an interview invite straight after you come back. Best of luck.