There are two ways to parse the noun phrase
Perfect day for a walk.
as a declarative sentence with ellipsis:
[It’s a] perfect day for a walk.
or as a nominal exclamative:
[What a] perfect day for a walk!
Linguists Paul Portner and Raffaella Zanuttini argue such noun phrases need not be parsed as omitting anything at all, but are a distinct type of exclamative in its own right.
This argument has logical appeal: such expressions are so common in spoken English that it seems superfluous to add something in analysis that is never present in the phrase itself. Put it back in and it becomes a different construction. The nominal exclamative can easily omit a determiner:
Great wedding! Nice car! Wretched weather today! Fantastic performance!
or include it:
[A] great crowd tonight! [A] long way to go yet! [A] beautiful dress she wore to the Oscars!
If these exclamatives are transformed into any other type: declarative (without ellipsis), interrogative, or a what-exclamative, then the article is obligatory according to standard rules:
Was there a great crowd tonight?
There was a great crowd tonight.
What a great crowd tonight!
An exclamative need not be spoken with great affect to be classified as such, and I am only using an exclamation mark to indicate that the expressions are not mere noun phrases fluttering in the breeze, but are complete expressions.
Given the special nature of this type of exclamative, it would be unwise to apply this omission of the indefinite article as some general rule of English usage.