I'm not sure when it became customary to applaud at the end of the day. I'd imagine it was to show thanks for all the hard work put forth in providing such a sumptous spirtual feast.
I did clap, because as someone who had been in a part and had family in a part too, it is alot of work and travel. What they want is ridiculous. There was one case where we rehearsed at a local KH, and one group drove two hours just to come and give their 15 minute part. I wondered how the CO could ask people to do that. Ridiculous.