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March past one had the next day? (5,4)

Ross

I believe the answer is:

april fool

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'day?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'march past one had the next' is the wordplay.
'march past' becomes 'april'.
'one had' says to put letters next to each other.
'the next' becomes 'fool' (I am not sure about this - if you are sure you should believe this answer much more).
'april'+'fool'='APRIL FOOL'

Can you help me to learn more?

(Other definitions for april fool that I've seen before include "Spring dupe!" , "I've been had" , "gullible person" , "butt of joke" , "Victim of practical joke at start of a month" .)

I've seen this clue in The Sunday Times and The Times.
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