neomutt sidebar calendar

calendar.rc

set sidebar_visible      = yes
set sidebar_width        = 22
set sidebar_divider_char = ""

named-mailboxes "      June 2023     " fake1
named-mailboxes "Mo Tu We Th Fr Sa Su" fake2
named-mailboxes "          1  2  3  4" fake3
named-mailboxes " 5  6  7  8  9 10 11" fake4
named-mailboxes "12 13 14 15 16 17 18" fake5
named-mailboxes "19 20 21 22 23 24 25" fake6
named-mailboxes "26 27 28 29 30      " fake7

How to abuse the sidebar.
You'll need some fake mailboxes, so touch fake{1..7} will do.