In Khmer, the words for Monday, Tuesday, Wednesday, etc., literally translate as Moon Day, Mars Day, Mercury Day, and so on. While it doesn't seem unlikely to name days after planets, it does seem odd that they would coincidentally line up in the same way. One obvious answer would be that when Cambodia was a part of French Indochina, the previous names of the week (or whatever system existed) was replaced with a system modeled after French. But if this were the case, why wouldn't the names of the week be t'ngai (Khmer for Day) lundi (French Monday), t'ngai martes, etc.
Is there a logical reason why the planets, sun and moon would be organized in the same sequential order?
The days are named after the Hellenistic astrological system, which in turn would have influenced the Indian names and then the Khmer names.
I can't answer your question, but FYI it's the same in Hindi: Somvar (I admit that one eludes me, but it definitely doesn't mean 'moon-day'), Mangalvar (Mars-day), Budhvar (Mercury-day), Guruvar (Jupiter-day), Shukravar (Venus-day), Shanivar (Saturn-day), Ravivar (Sun-day).