Voting is likely to take place at the earliest after lunch, but it may take time until late afternoon or even in the evening. After just over three years of delay and severe political throws, it may be time for a decision today.
But it is very unclear how it will end in the vote. Prime Minister Boris Johnson needs 318 out of 650 votes to be able to advance Brexit towards goals. The opposition will try to ensure that the agreement is stopped - or pushed further into the future.
Only four times before has the British Parliament gathered for Saturday's sitting: in September 1939 when World War II had erupted, in July 1949, in connection with the Suez crisis in 1956 and in April 1982 when Argentina invaded the British Falkland Islands.