When is the "happy anniversary" banner displayed?

Question

I have noticed a banner saying "Happy 10 year anniversary!" in the right sidebar on some sites recently - on Sports and on Academia.

Questions.

• Is this something which has to be requested by moderators or CMs, or is it added automatically by the Stack Exchange software?
• Is it a new feature? Or did I simply miss other sites which had a similar banner in the past?
• If it is done automatically, under which conditions is the banner added? (When does it start? When does it end? Are there also other banners in addition to the 10-year anniversary?)

Some results from the Wayback Machine and some related posts:

• I see that the Sports frontpage was without such a banner on March 2 and it was there on March 9. The site seems to have existed since February 8 (the first question, creation of the chatroom) and I see announcement of Public Beta on February 15.
• On Academia I see the frontpage without the banner on March 7 and with the banner on March 12. The site was created on February 14 (the first question, creating the chatroom). Looking at the posts mentioning public beta, the oldest one is from February 22.
• I see (from the posts on the per-site-meta) that such banner was displayed on Ask Ubuntu in 2020: Creating a banner for 10th birthday celebrations.
• This post from December 2021 mentions the anniversary banner on Russian Language and Movies&TV: The 10-year anniversary banner doesn't link anywhere by default; should it link to the blog post by default?
• Another post on a per-site-meta mentioning this banner was on Science Fiction & Fantasy: (How) should we mark the 10th anniversary of SFF.SE?
• There is a post on this site from a moderator (posted in September 2020) mentioning that several sites were/are close to 10-year anniversary: A lot of sites are going to be 10 this year - could we do something to celebrate?
• As pointed out in a comment, anniversaries like this are sometimes mentioned in a blog post - I see that these blog posts from 2021 contain a similar banner: Celebrating the Stack Exchange sites that turned 10 years old, Check out the Stack Exchange sites that turned 10 years old in Q3, Check out the Stack Exchange sites that turned 10 years old in Q4.

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(I wasn't sure about the choice of tags here. I have also included 10th-anniversary - I see that the tag-info says that it is about the anniversary of SO, but if the same anniversary is going to be celebrated in some way also for individual sites, maybe it could be reasonable to expand the scope of this tag.)

Answer 1

I believe that we are trying to do this for all sites that have 10th anniversaries, and can link the posts to meta announcements if the site mods want (the CM team handles that communication). These are put up manually (as in: I or a different developer copies in the html into a site setting for each site, and removes it when it is taken down, modifying it if it needs to link to an announcement post). We keep them up for about a month around the date of the anniversary (we batch them together, so it is not strictly "one month before or after" but rather "in the month of").