Good question by Andrew Sparrow on the Guardian Politics' live blog:
According to the House of Commons website, the Commons has only sat on Saturdays four times since 1939.
Three of those sittings were prompted by way or military conflict: 2 September 1939 (the day before the outbreak of the second world ward); 3 November 1956 (the Suez crisis); and 3 April 1982 (the invasion of the Falklands).
The other took place on 30 July 1949, when the last sitting before the summer recess was on a Saturday. (Anyone know why?)
(To these sittings must now be added 19 October 2019.)
Nothing seems to stand out in the Hansard for that day or the previous week.
A Wikipedia timeline suggests that Parliament passed the Legal Aid and Advice Act on that date, which establishes a much-extended system of Legal aid in England and Wales -- but the source given doesn't specify the specific day.
What happened on that day that was so important to get Parliament to sit on a Saturday rather than on the following week? Was it the aforementioned Legal Aid and Advice Act indeed? Was it a desire to just get on with the recess? Or was there something else?