Why did Lavan make a party for the men of Charan before the wedding?
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 7-:17): He told the men that he will swindle Yakov to stay another seven years. They desired this. He took securities from them not to reveal the ruse, and did not return them. The party was from their money!
Others explain that they secretly initiated a new law forbidding a father to marry off a younger daughter before her older sister (as Lavan will shortly explain to Ya'akov).