Where does “みそか” come from?

みそか (Japanese) comes from Japanese 日, from Old Japanese 日, from Proto-Japonic *pi — day, sun.

みそか (Japanese): the last day of a month

Definitions

みそか traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	日	Sunday
2	Old Japanese	日	the sun; sunlight, sunshine; daytime, daylight
3	Proto-Japonic	*pi	day, sun

Step	Language	Word	Meaning
1	Japanese	三十	thirty; thirty years of age
2	Japanese	三	three, 3; third; three
3	Middle Chinese	さん	—