dayday /deÉª/ âââ S1 W1 noun 1 24 hours [countable]TMC a period of 24 hours We spent three days in Paris. âWhat day is it today?â âFriday.â He left two days ago. Iâll call you in a couple of days.on a ... day Weâll have to hold the party on a different day.(on) that/the following/the previous day (=during a particular day) What really happened on that day so long ago? Over 10,000 soldiers died on that one day in January.