The last of Summer is Delight — A Midsummer Night's Dream

Enchantment's Syndicate is the key supported by Summer Delight. "To walk within the Vail" at the end describes Bottom's venture with an ass head. It's probably the poem's intention, an audacious venture within some veil to avoid a knock, like riddle poems.

The last of Summer is Delight－¹
Deterred by Retrospect.
'Tis Ecstasy's revealed Review－
Enchantment's Syndicate.

To meet it－nameless as it is－⁵
Without celestial Mail－
Audacious as without a Knock
To walk within the Vail.

(F.1380/J.1353)

[1] Summer: a hint on the answer. The last, Delight: the Pyramus and Thisbe at the end of the play by some craftsmen.
[2] Retrospect: a regard or reference to some fact, authority _{(OED 1a)}.
[2, 3] Deterred, Review: the craftsmen's rehearsal was deterred by Puck, who questioned their authority of Pyramus and Thisbe.
[4] Enchantment's Syndicate: Oberon, Titania, Puck and fairies.
[5] meet it: to meet this syndicate. nameless: that cannot be definitely named or described; inexpressible _{(OED 7)}. Craftsmen had no idea of what they were facing in the woods.
[6] Mail: a defensive covering, armor. Craftsmen had no celestial armor.
[7] Audacious: Nick Bottom the weaver, the only mortal who had met the fairies. Knock: a misfortune, a rebuff, a blow _{(OED 1b)}.
[8] Vail: Bottom's ass head.

PUCK: What hempen home-spuns have we swaggering here,
So near the Cradle of the Fairy Queen?
What, a Play toward? I'll be an auditor. ― A Midsummer Night's Dream