a period of 24 hours, especially from twelve o'clock one night to twelve o'clock the next night: January has 31 days. the days of the week every day He runs five miles every day. It took us almost a day to get here. the day before yesterday I saw him the day before yesterday. the day after tomorrow We leave the day after tomorrow. He was last seen alive five days ago. for days They haven't been seen for days (= for several days). in a few days I'll be seeing Pat in a few days. in a few days' time She says the protest will end in a few days' time. How's your day been? (= Have you enjoyed today?) Have a nice day! a big day I must get some sleep - I've got a big day (= an important day) tomorrow.