Something similar happened to me and I was furious as I had been very obliging for 3 Christmases.
The Department Head was playing favourites in my opinion and two other colleagues were going on mat leave shortly and I would be expected to cover some of their work.
I was pissed and was done being obliging.
I started looking about for a move as it was the push I needed, and when a good enough move fell into my lap I resigned.
I copied several other people besides my HoD and HR, and informed them I would be taking my considerable accumulated leave effect 4 days from my resignation, which was the beginning of December.
When my HoD's boss quickly asked for a word and asked what was going on, I explained to him that I wasn't accepting the unfair holiday allocation.
I dropped him completely in it over his behaviour, and because of it I would be taking all my leave.
The timing couldn't be worse for them but I didn't care, I was taking an extended holiday over the entire Christmas/NY period before I started my new job, which had been denied to me for the past 3 years.
It was a complete mess and the two women going on mat leave were relieved to be leaving it behind.
HoD was gone when they returned.