即日 (jí rì) is a formal, written word meaning 'this very day' or 'the same day' — it refers to the current day or a specific day already mentioned, often used in announcements and official notices. 当天 (dàng tiān) and 当日 (dāng rì) also mean 'the same day' but are more neutral in register; 即日 emphasizes immediacy and formality. 近日 (jìn rì) is different — it means 'recently' or 'in the next few days', not a specific day.