Sabbath

The Muslims observe their Sabbath on Friday, the Jews observe on Saturday and the Christians observe on Sunday. By the time Monday rolls around God is completely fuckin' worn out.

The Sabbath is a weekly day of religious observance (and often, as "rest"), found in all Abrahamic religions. The day commemorates the seventh day of creation; the day God is said to have rested: "And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made." (Genesis 2:2) The Sabbat&mdash;a term linguistically related to sabbath&mdash;is any one of several pagan celebrations of a similar nature.

Judaism
Of the Abrahamic religions, Jews follow the most doctrinal requirements for the Sabbath (Shabbat or Shabbos in Hebrew). In Judaism, each new day starts with the sunset, rather than the sunrise, so the Jewish Shabbat starts at sunset on Friday evening and ends at sunset on Saturday evening. Traditionally, observant Jews are prohibited from doing some 39 actions on Shabbat, ranging from farm work to lighting or extinguishing a fire, all based on the idea that one should not do any work on the Shabbat. Modern observant Jews have found many workarounds which allow them modern day luxuries without breaking Shabbat, including indirectly asking a goyim Gentile do it for them (since he can go to hell as long as he turns on our lights!) and the "Shabbat Setting" on the stove, which maintains a constant heat, but does not extinguish or ignite the entire 24 hours.

Jews generally do not impose the Shabbat traditions and laws onto Gentiles. Indeed, Jewish tradition states that a Gentile may voluntarily take on any commandment but Shabbat; therefore, non-Jews studying to convert to Orthodox Judaism must do something to break Shabbat every week until they finish their conversion. However, like the blue laws in the United States, the State of Israel does enforce basic Shabbat rules onto all businesses.

Christianity
Christians seem to subscribe to four different strains of thought regarding the Sabbath:

The Sabbath is Saturday, so we take that day off.
This is the position of the Seventh Day Adventists. It is basically the Jewish position, although Adventists aren't as strict; few Christians refrain from using electricity or lighting fires, for instance.

The Sabbath is Saturday, but we take Sunday off.
This appears to be the position of the Catholic Church. Roman Catholics hold that Jesus' sacrifice ended the obligation to honor the Sabbath. Worship of God on Sundays is a new, a distinct, obligation.

The Sabbath is Sunday, so we take Sunday off.
This seems to be the general Christian understanding, and it may result largely from lay people not understanding their own religion.

The Sabbath is a Jewish tradition. We're not Jewish.
Some Christians hold the position that the Sabbath is completely irrelevant to Christians, and Sunday is merely the day on which Christians tend to congregate.

 Clearly, someone's getting it wrong here, and whoever it is is going to pay for it.

Calculating the Sabbath doesn't always come easy, either. If you live near the, local authorities can change the boundaries and really mess up your 7-day-cycle Sabbath-keeping. And if you live off-planet (as in the fictional lunar colony in Robert A. Heinlein's novel The Moon is a Harsh Mistress, where concepts of Earth-based days and weeks can start looking rather artificial) you can go back to basics and conclude "that sundown Tuesday to sundown Wednesday, local time Garden of Eden (zone minus-two, Terra) [is] the Sabbath".

Forcible observation
Many blue laws are imposed on businesses to observe the Sabbath. The most common is a prohibition on the sale of alcohol.

Islam
Islam does not observe a day of rest but Friday is a holy day for Muslims when they try to do more religious observance and work less.