In a letter addressed to Brigham dated February 18, 1861, W. W. Phelps predicts that the slaves will have a "holiday as wonderful as their masters" by 1866.

Date: Feb 18, 1861
Type: Letter
Source: W. W. Phelps
LDS
Hearsay: Holograph
Direct
Reference: W. W. Phelps, Letter to Brigham Young, February 18, 1861, CR 1234 1, Church History Library
Scribe/Publisher: W. W. Phelps
People: W. W. Phelps
Audience: Brigham Young
Transcription: By 1866, these "slaves" will have a holiday as wonderful as their masters . . . sic transit gloria mundi [Thus passes the glory of the world]
Source Link: https://catalog.churchofjesuschrist.org/assets/279073c7-4a25-49a7-819b-f00d36895ab8/0/4
Citations in Mormonr Qnas: White Horse Prophecy