market day
A fixed phrase referring to the day on which a regular market is held, often in a specific town or area.
In this town, the 5th and 20th of every month are market days.
Compound of 市 (market) + が (subject marker) + 立つ (to be held, to take place) + 日 (day). The verb 立つ here carries the sense of an event being established or occurring.