Festivity is a noun.

The traditional afghan festivity had been banned during the taliban regime.

No, except perhaps in a poetic context. The reason that the adjective 'deft' can't be used to describe a 'festivity' is because deft describes an ability to perform with dexterity, cleverness, or skill. A festivity has no abilities and it can't perform; a festivity is a thing.

Suppose that the festivity was deftly achieved? The word 'deftly' is an adverb, modifying the verb 'achieved'.

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Yes it is