Refers to the latter part of spring, typically around April to May. Used in both noun and adverbial contexts.
Lured by the late spring weather, I went out for a walk.
third month of the lunar calendar
Obsolete term; historically referred to the third month of the lunar calendar, roughly corresponding to late spring in the modern calendar.
This flower blooms in late spring.
In the old lunar calendar, 晩春 referred to the third month.
晩春 refers to the tail end of spring, while 初夏 refers to the very start of summer, and the two are easy to mix up since they describe adjacent, similarly mild weather.
Compound of 晩 (late) and 春 (spring). The obsolete sense derives from the traditional lunar calendar.