The Mishnah states:
A besulah marries on the fourth day of the week.
What is the significance of the fourth day? The Gemara (Shabbos 19a; See Baal HaMaor Ibid) states that the three days following Shabbos are referred to as after Shabbos, whereas the three days that precede the Shabbos are referred to as before Shabbos. Perhaps our Mishnah alludes to this idea, as the Medrash (Bereishis Rabbah 11:8) states that Shabbos is the mate of the Jewish People. Thus, one should marry on the fourth day of the week or later in the week, as the marriage will be more connected to Shabbos, which reflects the marriage to the Jewish People.