Celebratory of baby to come — welcoming son (6)

I believe the answer is:

festal

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'celebratory' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for festal that I've seen before include "Relating to a holiday" , "Joyous, festive" , "Jovial" , "Relating to a festival; joyous" , "Jovially celebratory" .)

I've seen this clue in The Times.

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