This is 1121's latest announcement about the custom map event: You can find the download link in this tweet: https://twitter.com/akari1121aA/status/1341333157765890050. Timezone is JST.
Quote 1121 (auto translated) ()
CM Festival Pack Rollout! !! !! !! The #CM Festival starts at 20:30 on December 26th and 27th ! Take this opportunity to recruit #BF1942 #FHSW recruits and veterans to play with! Click here for December CMpack...
After this event there may be some newer versions or SSMs.