Tricky one this, as this is the first time the get-paid-early feature has been available over the Xmas period. It is usually (with a BACS payment) ‘get paid from 4pm on the business-day/week-day before you are supposed to’.
Going off the calendar for December, if you usually get paid on the 28th (Saturday), then you should get paid on the 27th (Friday) - which means you can claim from 4pm on Wednesday 26th. But the 26th is Boxing Day (UK Bank Holiday) and so it should roll forward to to the 25th, which is Christmas Day (another UK Bank Holiday), so in theory, you can claim from 4pm on the 24th. Happy Christmas!