Today, tomorrow and the days of the week

On day 18, 에 marked the time something happens: 여섯 시에. Most time words take it, including 주말 and all the days of the week. 오늘, 내일 and 어제 are the exceptions, because they already carry the 'on' inside them, just as English says 'tomorrow' and not 'on tomorrow'. Adding 에 to them is a common learner mistake; it is understood, but it sounds wrong.

The names come from the seven-day week of the sun, the moon and the five planets visible to the eye, the same scheme that gave English and French their day names. In East Asia the five planets are named after the five elements: 화성 fire is Mars, 수성 water is Mercury, 목성 wood is Jupiter, 금성 metal is Venus and 토성 earth is Saturn. So Saturday is Saturn's day in both languages, and Monday the moon's. To ask the day, say 오늘 무슨 요일이에요? (oneul museun yoirieyo?), with 무슨 meaning 'what kind of'.

Months are the most regular thing in Korean: take a Sino number from day 8, add 월, 'month', and you have it, all the way to 십이월, December. The two exceptions are about ease of saying. 육월 and 십월 would put a hard final consonant straight before 월, and speakers smoothed them to 유월 and 시월 long enough ago that the spelling now follows. Dates put the Sino number before 일, 'day', and the order is always year, month, day, largest first, just like the time on day 18.