tfl.gov .uk
says ...
"Hours of operation
Certain vehicles are subject to the Congestion Charge if driven within the charging zone between 07:00 and 18:00, Monday to Friday.
You do not have to pay the charge on weekends, English public holidays, designated non-charging days, or between 18:00 and 07:00.
Some vehicles or drivers qualify for discounts and exemptions.
Non-charging days
All charging days that fall between Christmas Day and New Year’s Day inclusive are designated non-charging days. Vehicles that drive within the zone on or between these dates will not incur the Congestion Charge".