Holiday on my return to work

Can anyone advise - my work only allow a certain number of people to have leave during Christmas. This is given on a first come first served basis and is requested in January. My maternity leave started in September 2024 and I plan to go back in October 2025 - my work are saying I cannot have this off as all of the slots have been allocated. Surely I should be allowed to have this time off as I wasn’t working in January, or was I expected to log in to my work system to request the time off? I am totally confused right now

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