A Sunday, generally near the time of the festival of the saint to whom the church is dedicated, when anciently it was customary to renew the rushes with which the church floor was strewed. The festival is still observed at Ambleside, Westmoreland, on the last Sunday in July, the church being dedicated to St. Anne, whose day is July 26. The present custom is to make the festival a flower Sunday, with rushes and flowers formed into fanciful devices. The preceding Saturday is a holiday, being the day when the old rushes were removed.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894