"I didn't even know she was ill until late Saturday night," Nicks wrote. "I wanted to be in London, I wanted to get to London — but we were told to wait."
Christine's family said in an announcement that the singer-songwriter was "in the company of her family" at the hospital Wednesday morning when she passed away "peacefully ... following a short illness."
Nicks described Christine as her "best friend in the world since the first of 1975" (when she joined Fleetwood Mac). She explained that since she learned of her fellow Rock and Roll Hall of Famer's illness "one song has been swirling around in my head, over and over. I thought I might possibly get to sing it to her, and so, I'm singing it to her now."
Nicks finished her note with some lyrics from Haim's "Hallelujah":
"I had a best friend but she has come to pass
One I wish I could see now you always remind me
That memories will last these arms reach out
You were there to protect me like a shield
Long hair running with me through the field
You've been with my all along
Nicks signed her note, "See you on the other side, My Love. Don't forget me. Always, Stevie."
In an interview given this past summer, Christine admitted that while she and Nicks were virtually inseparable on the last Fleetwood Mac tour, they had scarcely communicated since the tour ended.
Fleetwood Mac issued a statement Wednesday addressing Christine's passing:
"There are no words to describe our sadness at the passing of Christine McVie," the band said in a release. "She was truly one-of-a-kind, special and talented beyond measure. She was the best musician anyone could have in their band and the best friend anyone could have in their life. We were so lucky to have a life with her. Individually and together, we cherished Christine deeply and are thankful for the amazing memories we have. She will be so very missed."