Brewer's: Lætare Sunday
The fourth Sunday in Lent is so called from the first word of the Introit, which is from Isa. lxvi. 10: “Rejoice ye with Jerusalem, and be glad with her all ye that love her.” It is on this day that the pope blesses the Golden Rose.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894