Insert a new line in the "Next events" section in descending chronological order based on the start date of the event. Insert the information writing the line in the language matching the language that will be used during the event. Please note that, regardless the language you are using, the line has to be added in the English version of the page because the framework of the page is part of the translation system. Anyway you don't need to tag the line for translation, it will be presented with the same text in all versions. Please consider that administrators may take up to 24 hours to propagate the line to other language versions.
Write the line with:
event name (possibly with a link to the page in this or another wikiproject where the event is described);
address of the location where the event is held;
date(s) when the event is held.
After the event, move the line in the appropriate subsection of "Past events" in ascending chronological order.