Where "occurring" encompasses anything from flourishing or blooming to generally being active at the time, plus an astronomic sense of being descending at the time of sunset. Distinguished from crepuscular in that the latter can also be during the time around dawn as well. Borrowed around 1495 from Latin vespertīnus, from vesper, which was both evening and the evening star, better known as Venus when in the evening sky -- a cognate of Greek hésperos and English west. Note that the Christian service of vespers is held late afternoon or evening.
Datura is another vespertine flower.
Will you let me go upstairs and change into something a little more vespertine?
—Stephen Fry, Revenge: A Novel---L.