Why Is October Siwol, Not Sibwol, in Korean?

The original Korean readings of the characters 十 (ten) and 六 (six) are 십 (sip) and 육 (yuk). But when 월 (wol, "month") follows, the final consonant carries over and the words come out as [si-bwol] and [yu-gwol], which do not roll off the tongue easily. The common explanation is that dropping the consonant and saying 시월 and 유월 spread widely. Speech drifts toward what is easier to say like this, and when many people use a form for a long time, that sound can become the standard.

A sound that people have settled on in place of the original reading is called 속음 (sogeum), a customary reading. Article 52 of the Korean spelling rules says that when a Sino-Korean word is pronounced both in its original reading and in a customary one, each should be written as it is actually pronounced. For October and June the customary forms are standard, so they are written 시월 and 유월.

This happens more often than you might think. 초파일 (chopail), the name for Buddha's Birthday, comes from 初八日, and the final consonant of 팔 (pal, "eight") has dropped. 오뉴월 (onyuwol), which groups late spring and early summer together, also contains 六 in the form 뉴 (nyu). Even the fruit 모과 (mogwa, quince) was originally read 목과 (mokgwa).

Some words change their initial sound instead. 諾 is 낙 (nak) in 승낙 (seungnak, consent) but 락 (rak) in 허락 (heorak, permission), and 論 is 론 (ron) in 토론 (toron, debate) but 논 (non) in 의논 (uinon, discussion). All of these are written to match how people actually say them.

A common misconception is that every month changes. November is 십일월 (sibirwol) and December is 십이월 (sibiwol), written with the original reading. Dates keep it too, so the 10th is 십 일 (sip il); October and June are the only two that lose the consonant. Writing 십월 or 육월 in Korean letters is treated as a misspelling, which is why they show up often in spelling quizzes.

Writing the months as numerals, 10월 or 6월, causes no problem at all. It matters when you spell a date out in Korean letters, as on a wedding invitation, in a handwritten letter, or in song lyrics. You can write something like 「시월의 마지막 밤」 ("the last night of October"), and when reading aloud, just say [siwol] and [yuwol].