The session was another opportunity for the deputy groups to present their views on the law.
The decision comes after several days of uncertainty over the referendum date. The coalition initially proposed that it be held on 10 January to avert having the vote during the Christmas holidays.
But since this would be more than 45 days after the start of referendum proceedings, it would have required a two-thirds majority in parliament, which was seen as impossible.
The referendum refers to simple yet far-reaching amendments to the marriage and family relations act that redefine marriage not as a union between man and woman but between two consenting adults.
The parliament had attempted to stop the referendum, launched by a conservative Church-backed movement, but the Constitutional Court overturned the referendum ban.
The MPs did not debate only the date of the referendum today, but also expressed their views about the Constitutional Court decision and the law itself.
The opposition Democrats (SDS) and New Slovenia (NSi) are against the law, with Jelka Godec of the SDS saying that while the topic of same-sex couples should be approached seriously and responsibly, this should not be done with redefinition of marriage.
Simona Kustec Lipicer of the ruling Modern Centre Party (SMC) said that the decision of the Constitutional Court should be respected, although it was taken with a narrow majority. She wondered what message was being sent to same-sex partners with the referendum.
Coalition Social Democrats (SD) deputies are disappointed that this topic is debated in the first place. Slovenian society is not mature enough to understand that one cannot decide on human rights in a referendum, said MP Jan Škoberne.