Christmas Leave ?

I work in a dental practice that’s is open throughout Christmas but we are given only one day that we can work throughout Christmas. The rest of our leave will be unpaid.

We were not told that our leave between Christmas and new year for days unworked will be unpaid, they did not provide any notice that this would occur, this is not stated in our contracts because of this no one has saved any holiday entitlement to support our pay over the Christmas period. We were not aware that we would not get paid for forced closure/ days not being allowed to be worked. Also, our contracts not stating this at the time of even being employed ?

Is this illegal or can this be brought up to HR?