The flower of St. Peter who, along with St. Paul, is honored on this day, a national holiday in many countries and especially in Rome, according to Wilson's Almanac.
In French, yellow rattle is called Crote-de-Coq or Cock’s Comb (because the bloom resembles the wattle of a rooster?) which may be one of the reasons it is assigned to St. Peter, who denied Christ three times before the cock crowed.
Here’s a picture of yellow rattle and a short description of it. This is one of those plants that is an agricultural marker: when the seeds rattle, it’s time to cut the hay. It’s also a great plant for re-establishing hay meadows.
In medieval England, this was the day when rushes, or new-mown hay, were brought into the church to be spread on the floor, according to Kightly. Great carts of plaited rushes, decorated with flowers were brought in procession to the churches. Possibly an offering of thanks given for a successful hay harvest? Is hay being harvested where you live?
Kightly, Charles, The Perpetual Almanck of Days, Thames & Hudson 1987