星期日(xīng qī rì) - Sunday in Mandarin

星期日 (xīngqī rì) is Sunday, breaking the number pattern the rest of the week follows: instead of a digit, it takes 日 (rì, "sun, day"), the same character that names the day itself in formal writing.

星期日 (xīngqī rì) is the more formal, more written of the two Sunday words - the one printed on official calendars, tickets, and forms - while its twin 星期天 (xīngqī tiān) leans more colloquial in speech. Both are common and either is understood everywhere.

As a time word it behaves like any other weekday: 星期日我们去教堂 (xīngqī rì wǒmen qù jiàotáng), "we go to church on Sunday", sitting in front of the verb with no preposition attached.