The etymology of Czech names of the days are quite easy to guess. Most are just numbers (cf. úterý and Russian второй, čtvrtek, pátek), středa is in the middle and sobota is obviously from sabbath.
In Polish we have "niedziela" and "poniedziałek" but etymology is not so obvious because we don't use the word "dělat" as often. We have "działać" (to act) but "robić" has a closer meaning to Czech "dělat". I would think about "dzielić" (to divide) too.
In Czech this connection is much more obvious.