Special day for old Roman teams (5)

I believe the answer is:

sides

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'old roman teams' is the definition.
'sides' can be an answer for 'teams' (eg football sides). I'm not certain of the 'old roman' bit.

'special day' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'day' becomes 'ides' (ides is a kind of day).
's'+'ides'='SIDES'

'for' is the link.

Can you help me to learn more?

(Other definitions for sides that I've seen before include "Flanks, or opposing teams" , "More than one team" , "parties" , "Angles" , "camps" .)

I've seen this clue in The Guardian.

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