Why did Lavan make a party for the men of Charan before the wedding?
Targum Yonasan: To take counsel with them how to trick Yaakov into remaining in Charan another seven years (Refer to 29:22:1:3*). They counseled him to give Leah to Yaakov instead of Rachel.
Da'as Zekenim, Hadar Zekenim: He wanted to get Yaakov drunk, so he would not distinguish Leah from Rachel. This is why he did not make a party for Rachel!
Tosfos ha'Shalem (2, from Bereishis Rabah 70:19): He told the men that he will swindle Yaakov to stay another seven years. They desired this. 1 He took securities from them not to reveal the ruse, 2 and did not return them. The party was from their money!
Oznayim la'Torah #1: So that the people of the town would assist him to trick Yaakov by switching Leah for Rachel. 3
Oznayim la'Torah #2: In order to be able to subsequently tell Yaakov that he did not give him Rachel because the townspeople would not let him.
Why did Lavan gather all the people of the place?
Malbim #1: It was to agree to make a custom not to marry off a younger daughter before her older sister.
Malbim #2: Yaakov would be ashamed to divorce Leah, due to publicity.
Malbim #3: Yaakov would not check that it is not Leah. He would rely on Lavan not to swindle him like this in front of the entire city.