Prayers in the House of Commons
At the start of the day there are prayers in the House of Commons. This is more important to MPs because if you pray you can book a seat on the benches for the day.
Any time after 8am when the chamber opens an MP can, themselves and not on behalf of another MP, collect a little green card, write their name on it and slot it into a little slot on the green benches.
Then at prayer time (when the business starts) they arrive. The card in on the bench. They put it back into the slot and pray. If they are not there at that time they lose their reservation.
Wednesdays is a day upon which lots of people wish to reserve a seat you do, therefore, see far more people praying.