(nowhistorical) A Sunday when a church's relics are especially venerated.
c. 1541, The Chronicle of Calais, London 1846:
The 23. of Henry the Seventh, the 9. of July, beinge relyke sonday, there was sene at Calleys an innumerable swarme of whit butterflyes cominge out of the north-este [...].
On Relic Sunday, an annual day whereon all the canons, clerks, and singing men were wont to go round the church in procession, every one carrying a relic, Testwood's offences rose very high.