Why did Lavan make a party for the men of Charan before the wedding?
Targum Yonasan: To take counsel with them how to trick Yakov into remaining in Charan another seven years (Refer to 29:22:1:3*). They counseled him to give Leah to Yakov instead of Rachel.
Da'as Zekenim, Hadar Zekenim: He wanted to get Yakov 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 Yakov 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!
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: Yakov would be ashamed to divorce Leah, due to publicity.
Malbim #3: Yakov 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.