It's time to be serious; carefree times have ended. For example, Now that he's been promoted the party's over; he has to write a report every week. This expression uses party in the sense of “a pleasant social gathering.” [c. 1930]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.