“If 1520 in Everett is reasonable, it makes me wonder why the 810 in Ephrata and the 680s in Olympia and Omak are daytime only.”
Nighttime operation must pass interference analysis which involves skywave calculations which
are not only quite complicated but involve so many different factors it’s not inconceivable that
1520, which is almost half the wavelength of stations on the left half of the dial, might pass muster
especially since Everett isn’t that far from Oregon City (far enough away for groundwave clearance
but not far enough away to have a skywave problem).
It does depend on what kind of antenna configuration you propose and a whole host of factors too
numerous and complex to try and discuss here. Probably the most influential factor is who else is
on (or adjacent) to the frequency and where they are located. Those lower frequencies probably
have some pretty significant amounts of skywave and without doing a complete channel study
it’s hard to even guess why they remain daytime only beyond the above interference consideration.