The Jewish people didn't believe that Jesus Christ was the true Messiah. They thought that he existed and was a good man, but was not the son of G-d. For this same reason they don't celebrate Christmas, Good Friday, Easter and any other holiday involving Jesus.
The answer completely depends on the Reform Jew you are talking about. Orthodox Jews follow all of the laws (no driving, cooking, etc) and some Reform Jews do the exact same thing.
There are some Christians in Israel who do celebrate Easter, but the Jews and Moslems in Israel do not celebrate it.
Any marriages on Shabbat are strictly prohibited by Orthodox and Conservative Jews, and by reason of custom, completely discouraged by Reform Jews. Most Jews that marry on Saturday are secular Jews, and therefore would not be married during a prayer service.
They pray, eat, pray, and eat some more. And sit around and shmooze a lot.
they dont celebrate christmas
Some countries in history banned the practice of Judaism.
Orthodox Jews generally refrain from using electronics on Shabbat, which begins at sundown on Friday and ends at nightfall on Saturday. This is because operating electronics can be seen as a form of "work" or violation of the prohibition against creating fire on Shabbat. Some Orthodox Jews may use certain pre-set devices that are permissible according to Jewish law.
The question as written is confusing.Religious Jews do not celebrate Christmas, so when it falls is completely irrelevant to any other extant Jewish celebration. Non-Religious Jews, some of whom celebrate Christmas in violation of Jewish beliefs on such, are usually not Shabbat-observant, so the restrictions of Shabbat that would directly interfere with Christmas preparations are irrelevant.
Saturday is the Sabbath in Judaism. Orthodox (and some non-orthodox) Jews will not work. Many non-Orthodox Jews will work, but still remember the Sabbath day in their own ways. (And yes, there are some Orthodox Jews that work on Saturdays, but they do not advertise this fact).
Reform Jews have only 1 seder instead of 2.
At a shabbat service at your home or at a synogauge you may light the shabbat candles and say the shabbat blessings. Also some people follow it with hallah and/or a kiddush.
Some certainly only celebrate Rosh Hashanah because they feel guilty otherwise, but most Jews choose to celebrate Rosh Hashanah because they want to engage with their history and traditions.