A source close to the family tells E! News that fairly extreme measures were taken to keep the grand occasion a secret from the 33-year-old, as well as her sisters.
"Only Kris knew it was a proposal. Her sisters didn't know," the insider explained. "They just thought that it was a birthday surprise."
So you can imagine how shocked they were when they arrived at San Francisco's AT&T Park and learned of the real reason they were asked to attend.
"Kourtney, Khloé, Kris and Kendall were all scheduled to be in Arizona for a spa getaway for a few days. The word came down on Friday that Kanye wanted to surprise Kim on her birthday…so the Arizona trip was canceled. But everyone told Kim they were still going to Arizona," the source added.
Those clever Kardashians!
Khloé Kardashian Odom and Kylie and Kendall Jenner all tweeted and posted pictures on Instagram in an attempt to throw the soon-to-be Mrs. West for a loop.
"Kim really had no idea. Khloé even talked to Kim on the phone that afternoon and pretended she was in Arizona, but she was actually in her hotel room in San Francisco."
And clearly their secret plan went off without a hitch—pun intended.
Approximately 100 people took part in the engagement celebration Monday evening to help make the night unforgettable.
Kim exclusively spoke with E! News just a day after saying, "Last night was truly magical!!! I am the luckiest girl in the world! I get to marry my best friend!"
Congrats to the future husband and wife!