Hi Robby, thanks for taking the time to report this!

I wonder if you can edit [[File:Wiki.png]] locally. Not sure though.

In any case, please add the URL of your local consensus to the "URL" bug field of this report.

InitialiseSetting.php says

'lbwiki' => '//upload.wikimedia.org/wikipedia/commons/9/9f/Wikipedia-logo-v2-lb.png',

in line 503 so that I believe this can indeed be resolved without a config change.

https://commons.wikimedia.org/wiki/File:Wikipedia-logo-v2-lb.png would need to be replaced by the requested logo during said timespan then.

While the method suggested by Vogone in comment 2 will definitely work, it is generally suggested that wikis use CSS to overwrite their usual logos for occasions like that.

You can do so by copying the CSS added in this edit https://pl.wikipedia.org/w/index.php?diff=next&oldid=37560298 to your local Common.css file.

Of course, you'll need to change the logo to the one you're going to use for your celebrations; I also suggest that you contact a Wikimedia Commons administrator to protect the file in question for the duration of the celebrations to prevent vandalism.

With that said, I'm closing this as RESOLVED INVALID as you are able to do this without the involvement of Bugzilla.

Thanks for asking, though :-)

Many thanks for all these information. I've already successfully tested the solution provided in Comment 3 by Tomasz W. Kozlowski. Best regards from sunny Luxembourg.