The Maseches begins with the words besulah nisas leyom harevii. It is noteworthy that we commence this Maseches in the month of Elul, which is the mazel of besulah. This reflects the idea stated in the Medrash (Vayikra Rabbah 29:12) that on Rosh Hashanah, we are deemed to be a new creation. Hashem should allow us to start the year with freshness and we should be rejuvenated to study Torah with more diligence and perform mitzvos with more focus.
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.