What kind of poem is "Ode to Evening"?

William Collins's "Ode to Evening" is an ode, as noted in the title. An ode is a poem in which the speaker celebrates or appreciates something specific, in this case, the evening. Some other famous odes include Keats's "Ode on a Nightingale" and "Ode on a Grecian Urn," both of which meditate on and praise the object noted in the title, as Collins does in his poem.
The ode is addressed to evening in apostrophe, which is a poetic address to something that is either unable to respond (like an inanimate object or part of nature) or not present. The ode primarily describes the calm of evening. For example, the speaker writes,

Now teach me, maid composed,
To breathe some softened strain,
Whose numbers stealing through thy dark'ning vale
May not unseemly with its stillness suit,
As musing slow, I hail
Thy genial loved return. (15-20)

The speaker seeks the wisdom of the evening and wants to be more like the evening, "composed" and "still." The speaker celebrates the return of evening and is happy in its presence.

Further, the speaker describes evening as a magical time:


For when thy folding star arising shows
His paly circlet, at his warning lamp
The fragrant Hours, and elves
Who slept in flowers the day,
And many a nymph who wreathes her brows with sedge
And sheds the fresh'ning dew, and lovelier still,
The pensive pleasures sweet
Prepare thy shad'wy car. (21-28)



The evening allows for the "elves" and "nymph[s]" to emerge. The speaker looks forward to "The pensive pleasures" that arrive with evening, thanks to these otherworldly sprites.

The second and final stanza of "Ode to Evening" relates evening to the seasons. Evening must undergo the moods of the seasons, dealing with all of the natural phenomena associated with each time of year. Nonetheless, evening remains a source of joy and inspiration to the speaker:


So long, sure-found beneath the sylvan shed,
Shall Fancy, Friendship, Science, rose-lipp'd Health,
Thy gentlest influence own,
And hymn thy fav'rite name! (49-52)



Evening will still have its "gentlest influence" throughout all four seasons. There is always much to celebrate when it is evening. The speaker writes a true ode, one that expresses his emotional and joyful appreciation of evening.