I think I figured it all out now. The 2 spring DLCs we got in the past, both released on a tuesday within the same weak as easter sunday, meaning this is of big importance for sales. If they were to go by the same scheduling, then we can expect the DLC to release on 12.4. (17.4. being easter). Now based on the SEA animal pack timeline, since we expect this to probably be an animal pack, the new feature reveal should be next thursday on the 24.3. then.
However I have to note that this has been the biggest drought we faced ever since covid started (SA pack to Australia was 20 weeks) (if my predictions are right, then EU pack to mystery DLC will be 17 weeks). The difference between the last two winter and spring DLCs has been 16 weeks in both cases.
With such a long distance and development time, my personal hopes for an aviary pack are pretty high now, because spring would be the most fitting season, and that would totally make all the wait worth it for me. Remember that it last years spring update that announced the new format of animal pack DLCs, something very unexpected!