A Good Old Fashioned Orgy

Eric (Jason Sudeikis) is a perpetual adolescent who lives to party, holding lavish theme-events with his friends using his father's large house in the Hamptons. On July 3, Eric throws a white trash bash party attended by his friends Sue (Michelle Borth), Adam (Nick Kroll), Mike (Tyler Labine), Laura (Lindsay Sloane), Kate (Lucy Punch), Glenn (Will Forte), Doug (Martin Starr) and his girlfriend Willow (Angela Sarafyan), Alison (Lake Bell) and her boyfriend Marcus (Rhys Coiro). Eric meets Kelly (Leslie Bibb) at the party. The following morning, Eric's father (Don Johnson) arrives and informs him that he is selling the house. The next weekend, the group arrive back at the house and find it for sale by Dody (Lin Shaye) and Kelly.
Eric and Mike decide to throw a last ultimate party at the house on Labor Day weekend. Lamenting the more liberal sex attitude of the younger generation that passed over his own, Eric suggests having an orgy. When they present the idea to their friends, they are all reluctant, but when Mike and Eric argue that afterwards everyone will be moving away from each other and that this might be their last real time all together, Sue agrees to join. Laura joins the plan to improve her self-esteem, and is later joined by Alison after she breaks up with Marcus. Sue has feelings for Eric from high school, but Eric pursues a relationship with Kelly, to attempt to interfere with her selling the house. Doug and Willow join the orgy, hoping it will make him more confident. At Kate and Glenn's wedding, a drunken Adam reveals he has lost his job because Eric destroyed his phone at the White Trash bash, and agrees to join the orgy. Glenn finds out about the orgy and informs Kate, causing the pair to fall out with the group for not including them.
Eric and Mike go to an underground sex club to research orgies, and gain advice from Mike's uncle Vic (David Koechner). Later, during a date with Kelly, she asks Eric to meet with her friends on Labor Day, to which he reluctantly agrees. Eric admits to Mike that he is developing feelings for Kelly and is no longer sure about taking part in the orgy. The week before Labor Day, Kate and Glenn ask to join the orgy but the group refuses because the couple have a child. Kelly tells Eric that she has asked Dody to slow the house sale down until the end of the summer.
The Labor Day weekend arrives and the group prepare for the orgy. Doug decides to back out at the last minute, causing the group to descend into arguments, culminating when Alison publicly reveals Sue's feelings for Eric, and Doug accuses Mike of being Eric's "pet". Glenn and Kate arrive uninvited, intending to gatecrash the orgy, but they end up having sex in their car. Eric decides to leave and go to Kelly's home, but discovers she is on a date. Eric returns to the party where the group reconcile, and the orgy finally begins.
On Labor Day, Doug gives Mike a copy of his finished album, having finally found the confidence to pursue his music career. Sue gets over her feelings for Eric, Adam and Laura enter into a relationship, and Eric reconciles with Kelly. Kelly tells Eric that the house deal fell through and it will now take longer to sell. Eric begins planning a party for Memorial Day.

Question: When they decide to celebrate the party in the movie?
Answer: Labour Day Weekend