office day; in-office workday
A day when one physically goes to the workplace, as opposed to working from home or taking a day off. Common in hybrid work contexts.
My office days this week are Tuesday and Thursday.
On office days, I have to leave home early in the morning.
テレワーク is a broader term for remote work, while 出社日 specifically refers to the day one commutes to the office.
Compound of 出社 (しゅっしゃ, 'going to the office') and 日 (び, 'day').