How do you say “Day off” in Japanese?「休日(きゅうじつ)」Kyūjitsu
"Holiday" is「休日(きゅうじつ)」or「休みの日(やすみのひ)」in Japanese. 「(私は)休日にピアノの練習をします。」means "I practice playing the piano on my days off."「次の休みの日に椅子を買いに行きます。」means "I'm going to buy a chair on my next day off."