Stupidly, a Tory disengages when England celebrates (5,7,3)

I believe the answer is:

saint georges day

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'england celebrates' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'stupidly a tory disengages' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'a'+'tory'+'disengages'='atorydisengages'
'atorydisengages' with letters rearranged gives 'SAINT GEORGES DAY'.

'when' is the link.

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I've seen this clue in The Independent.

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