What, where and when annual conferences are held to develop the LaTeX?
The major TeX yearly TeX conferences are
The TUG meeting (http://tug.org/tug2021/ is the next): As TUG is both the international TeX group and also the US national group, the meeting is held most often in the US but does move around. (For 2020 and 2021, COVID-19 has meant it is taking place virtually)
The BachoTeX meeting (http://www.gust.org.pl/bachotex) in Poland
The ConTeXt user meeting (https://meeting.contextgarden.net/), which takes place in Europe
The DANTE Spring and Autumn meetings (https://www.dante.de/): formally for the German user group
The GuIT annual meeting (https://www.guitex.org/home/meeting?lang=en): again, formally a national user meeting (Italian users)
Worth adding is that these are not strictly developer meetings: the topics vary widely.