A bit esoteric this one, but the logic goes something like this:
This is the dedication of the ‘top’ or ‘first’ church in Rome, which is remembering the beginnings of the Catholic church, which reminds us of the whole catholic church, and that it is the people being church, not the buildings, that matter (this is a bit garbled, sorry).
so this is a feast day for the ‘family’ of the Roman Catholic Church.
Hooray for: church, catholics, the pope
Music: Christus Vincit, I suppose, or The Church’s one foundation, or a nice bit of plainsong
Food: Rock cake (hmm, that’s an idea)
Visit: any catholic church
Prayer: the Mass