Saturday and Sunday; weekend
Refers specifically to the two-day period of Saturday and Sunday, often in contexts like days off, plans, or schedules. In Japan, the typical weekend is Saturday and Sunday.
土日はいつも家族と過ごします。
I always spend Saturdays and Sundays with my family.
I went to see a movie with a friend on the weekend.
この店は土日も営業しています。
This shop is open on Saturdays and Sundays as well.
This company is closed on Saturdays and Sundays.
I plan to use the weekend to go on a trip.
平日 means the weekdays (Monday to Friday), the direct opposite of 土日, which refers to Saturday and Sunday.
Compound of 土 (ど, 'Saturday') and 日 (にち, 'Sunday').