The reason this is hard to understand is because it is actually by Richard Hooker, from his work "Of the Lawes Ecclesiastical Politie", published in 1594,
and also because Walter de la Mare has chosen to quote it, but not in full.
The original reads:
"; but the days which are chosen out to serve as public memorials of such his mercies, ought to be clothed with those outward robes of holiness, whereby their difference from other days may be made sensible. But because time in itself, as hath been already proved, can receive no alteration; the hallowing of Festival-days must consist in the shape or countenance which we put upon the affairs that are incident unto those days."
"Hallowing" means to make holy. "All Hallows Eve", is the old name for Halloween on 31st October, because All Hallows, or All Saints' Day, follows it on 1st November.
"Incident", in this archaic context, refers to something happening or occurring.
I take it all to mean that "Because time itself cannot be changed, the sanctifying of holy days must arise due to the form, or approval, which we give to the affairs which happen on those days".
Or to paraphrase:
"Holy days are not holy in themselves, but are made so by the actions which we perform on them."
All Hallows is set in a cathedral, which I presume may be one of the reasons why Walter de la Mare chose this quotation.
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