if the takanah is to get married on wednesday why doesnt the mishnah only tell me that beis din comes to session on thursday, why bother telling me about monday
The Mishnah may be hinting to the fact that the Takanah applies only where Ezra's Takanah of Monday and Thursday was practiced, and not where Beis Din sits every day (Gemara 4a).
If it had said "because Beis Din sits on Thursday...," we would have thought that even in places where they sit other days as well, the Rabanan already instituted a Takanah and said "Lo Plug," you must get married on Wed. The Mishnah therefore hints that the Takanah, from the start, was only instituted for places where Beis Din meets only on Monday and Thursday.
(See Shitah Mekubetzes at the end of the Mishnah, who finds a different inference in the Mishnah to this Halachah -- from the word " Hayah Mashkim.")
-Mordecai Kornfeld