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I understand that 上旬 means the first 10 days of the month, and that 下旬 means the last (approximately) 10 days of the month. However, I would like to understand the underlying logic for the choice of 上 and 下 for the beginning and end (respectively) of the month.
After all, the later days of the month have higher numbers (21 22 23 etc), so isn't it counter-intuitive that they labeled with 下? There must be a different logical interpretation...
In my comment above, I answered a bit tongue-in-cheek-ily that calendars have the start of the month at the top, but if you want a concrete reference for the meaning of it, one of the meanings of 上 is 順序が先のほう (source). Another example of a word that uses this meaning is 上巻 "first volume" (of a manga series, etc.).
