Microsoft has announced that they will not be releasing cumulative updates previews in December 2020 due to limited staff and operations during the upcoming holidays. Microsoft associates each week of a month with a letter ranging from ‘A’ to ‘D’ The ‘B’ release is classified as ‘Important’ and automatically installed by Windows. Microsoft also releases cumulative update previews on the ‘C’ and ‘D,’ weeks that contain fixes being released in the next Patch Tuesday. Standard Windows servicing will resume with the January 2021 security releases.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-pauses-windows-cumulative-update-previews-for-december/