The Mishna ruled that one cannot marry a wife on Shabbos or Yom Tov. This was included in the second category of actions that are not mandatory mitzvos. The Gemora asks that marrying is indeed an obligation since this will enable him to fulfill the mistva of having children. The Gemora answers that the Mishna is referring to a case where he previously had a wife and children.
Tosfos states that even if one never had children, it is still forbidden to marry on Shabbos. The reason for this is explained in the Gemora later that we are concerned that the groom will write the kesuvah on Shabbos. Tosfos cites a Yerushalmi who offers a different reason for not marrying on Shabbos. The Yerushalmi learns that it is forbidden since one is not permitted to acquire objects on Shabbos. Rabbeinu Tam disagrees and holds that one who does not have children is permitted to marry on Shabbos. The Ran quotes Rabbeinu Tam that this permission is limited to extreme cases and generally it is forbidden.
The Rama in teshuvos (125) relates an incident that happened where a father arranged for his daughter to get married and passed away suddenly days before the wedding. An uncle had compassion on her and brought her into his house. There was no money for her dowry and the neighbors had to convince the girl to continue with her marriage plans. The chasunah was scheduled for a Friday afternoon and they were still short a substantial amount of money for her dowry. The two families began to fight and the groom was contemplating breaking off the arrangement. Many people interfered and peace was restored one hour after nightfall.
The Rama decided that the chasunah should take place on Shabbos since he was worried that if they would wait until after Shabbos, the peace would be lost. There were many people that attacked the Rama for transgressing the prohibition of marrying on Shabbos. The Rama defended his position stating that he relied on Rabbeinu Tam that a marriage can take place on Shabbos in extreme situations.
Furthermore, he stated that the reason for the prohibition is because we are concerned that the groom will write the kesuvah on Shabbos and since the custom became that the groom does not write the kesuvah himself, there is no reason to be concerned.
It is well known that from that incident, it became the custom in the city of Crakow not to arrange chasunos for Friday.