It depends on the circumstances. On the face of it it's disgraceful but without the facts we can't tell.
For example, if it's a £1000 a month flat and last year's increase was £10, this year's would be £30 which is a 3% increase.
The timing for Christmas Eve might be because the OP signed a six month contract on 24th June and the landlord can only increase the rent when there's a contract renewal.
(Or they're just greedy bastards, which is more likely.)