The first line of "Elegy Written in a Church Courtyard" indicates that it is evening. This is when the curfew tolls the bell (knell) of the parting (sunset) day. The first line of the next stanza also notes the glimmering landscape fades and this means that as the sun sets there is less and less sunlight reflecting (glimmering) off of the land.
Line 17, "The breezy call of incense-breathing Morn," makes it sound like the time has shifted to morning, but this just implies a pun on "mourning" because he's talking about the forefathers in their graves (lowly bed).
Evening is also symbolic of death. This is a common symbol in poetry; the end of a day or the end of a season symbolizing the end of a life. The speaker also stresses the "solemn stillness" of the evening in line 6, indicating a somber, contemplative, respectful tone.