Each one contains a chocolate shaped like a famous spacecraft and, for the later numbers, a pamphlet on managing anxiety.
This comic depicts a special advent calendar: instead of celebrating the Christian Advent by counting down to December 25th, it instead celebrates the launch of the James Webb Space Telescope by counting down to December 22nd.
At the time this comic was published, the James Webb Space Telescope (JWST) was scheduled to be launched on the 22nd of December, 2021 (after many prior delays). Christmas would indeed have come early for astronomers if the launch had been successful and on time. By December 14, the launch date had been pushed back again to "no earlier than December 24", as NASA was working on resolving a communications issue between the observatory and its launch vehicle system. This was followed by another delay announced on December 21, when the launch date was pushed back to December 25, due to weather concerns. It was successfully launched from Kourou in French Guiana on December 25 at 09:20 FGT (12:20 UTC, 07:20 EST), as hoped for in this comic: 2559: December 25th Launch.
A normal advent calendar marks the days until Christmas by allowing miniature doors to be opened, or other means of revealing some treat/picture. This is often from the 1st of the month until the 'big reveal' on the 24th or 25th, though other schemes may exist in other cultures. This particular calendar features 18 hexagonal features, intended to be sequentially accessed over several days, in the same layout as the 18 gold-beryllium mirror segments designed to fold out to form the JWST's primary mirror. The first door is on the 5th, two days after this comic's publication date, making the last on the 22nd, the 'Big Day'.
Cueball's question could be interpreted two ways: Cueball doesn't know about JWST, so he is asking why this advent calendar ends before Christmas (and possibly fearing this calendar is similar to the one in 1245: 10-Day Forecast); or Cueball does know about JWST and its history of delays, so he is asking why the calendar ends on 22 when there is no certainty in that launch date (and also implying that he expects it to be delayed). [Note: two weeks after the comic was posted, the JWST was again delayed, this time to no earlier than Christmas Eve (and later finally to Christmas Day itself), making the expectation accurate. This would also make a traditional advent calendar serve equally well, were it not for the hexagon design.]
The title text references the fact that chocolates in advent calendars are often molded into different shapes, and the fact that the later numbers have a "pamphlet on managing anxiety" is probably supposed to quell the impeding fear that the launch could be delayed further or go wrong. The telescope's launch was initially planned for 2007, but due to various redesigns, financial issues, accidents, flaws, and the COVID-19 pandemic, the launch date was pushed back to 2011, then 2013, 2018, 2020, May 2021, October 2021, and finally to the current launch date in December 2021. It may also allude to post-launch concerns; even if the launch goes well, there will still be nervousness about the complex 160-day process in which the JWST reaches its intended observation point 930,000 miles from Earth, many subsystems are unfolded/deployed, and the instrument passes its final calibrations. There is effectively no way to rescue/repair this expensive piece of equipment should anything be amiss, unlike the Hubble Space Telescope, which was visited five times by Space Shuttles to remedy and enhance various features. (There exist issues with even Hubble that cannot currently be considered repairable without the Shuttles or any proven replacement, and the JWST will be located far beyond Hubble's operational orbit in a place much more difficult to get to.)
The JWST has been referenced previously in 1730: Starshade, 2014: JWST Delays and 2447: Hammer Incident, mentioned in 1461: Payloads as well as indirectly in 975: Occulting Telescope. After this comic it was referenced in 2559: December 25th Launch and 2564: Sunshield.