When Iris is on the plane heading to L.A., she receives an email from the night before from Jasper - and the phone has the date and time at the top: December 18 at 11 in the morning. 

Iris’s boss also comments that they aren’t closing for the holidays just yet, but having the party before most of the staff heads out for their holiday. This indicates that the party is actually on December 17 (which was a Sunday. Weird but maybe).