for a first point of view, there were no common ground with the spanish or english equivalents. That ass why I started to do some research.
I came up with the origin of the names. And for my good surprice, there is a lot in common.
The spanish and english names are based on the name of the planets, the same goes to the japanese names... The only point when I got confused and have to invest more in is the origen of the names of the planets in japanese... doing so, the days of the week will be mastered in no so much time, because tuesday (tor-Mars-fire), in japanese is kayoubi when ka is fire and easily related to Mars. But wednesday (from german good Woden, is related to the planet Mercury), but that is the japanese water planet, that is way wednesday is in japanese suiyoubi, the water day.
Thas is why, if I get to know the relation between the japanese name of the planets and the planets itself, I could finish this list and understand it (the base of knowing ;) ).
nichiyoubi -sunday - domingo (nichi = sun = sol )
getsuyoubi - monday - lunes (getsu=moon=luna)
kayoubi - tuesday - martes (ka=fire=fuego) [Marte/Nordic god Tyr/fuego]
suiyoubi - wednesday - miercoles (sui=water=agua) [mercurio/Woden english god for mercury/agua]
mokuyoubi - thursday - jueves (moku = wood = madera) [jupiter/Nordic god thor/madera]
kinyoubi - friday - viernes (kin = Gold/Money = Oro/Dinero)[venus/Frigg,germanic translation on venus/metal]
doyoubi - saturday - sabado ( do = Earth/Soil = Tierra/suelo)[dia de saturno/ de descanso (hebreo shabat)/tierra)
learning this way is quite funny (divertido).
by the way, I post a more detailed explanation at http://fiubando.blogspot.com/2009/05/sobre-los-dias-de-la-semana.html in spanish. And I found http://www.livemocha.com/, it is not the same as lang-8, but it is a nice complement for this special lovely site.