Gather at Guildhall Yard for about 2pm for a Harvest Festival procession to the host church, St Mary-le-Bow, where the service begins at 3pm. There will be seasonal hymns and an address from Harry Mayhead, the Pearly King of Bow Bells. Many people choose to wear period costume, although that is by no means obligatory.
Note: the service is always standing room only - if you want to attend the service, it might be worth skipping the procession and heading straight to the church instead.