middle day
Refers to the middle day of a multi-day event, especially a sumo tournament or theatrical run. Not used for general 'middle of the day'.
The middle day of the grand sumo tournament is always crowded.
この芝居の中日は、特別な演目が組まれる。
On the middle day of this play's run, a special program is arranged.
Compound of 中 (なか, 'middle') and 日 (ひ, 'day'), with rendaku voicing to び. The reading なかび is specific to this meaning.