When even was come (opsias genomenēs). Genitive absolute. Not sunset about 6 p.m. as in Mat 8:16 and as in Mat 14:23, but the first of the two “evenings” beginning at 3 p.m.
The place is desert (erēmos estin ho topos). Not a desolate region, simply lonely, comparatively uninhabited with no large towns near. There were “villages” (kōmas) where the people could buy food, but they would need time to go to them. Probably this is the idea of the disciples when they add:
The time is already past (hē hōra ēdē parēlthen). They must hurry.