https://en.wikipedia.org/wiki/International_Congress_of_Mathematicians.

Started in 1897, so long running.

As for the numbering:

In the aftermath of World War I, at the insistence of the Allied Powers, the 1920 ICM in Strasbourg and the 1924 ICM in Toronto excluded mathematicians from the countries formerly part of the Central Powers. This resulted in a still unresolved controversy as to whether to count the Strasbourg and Toronto congresses as true ICMs ... As a consequence of this controversy, from the 1932 Zürich congress onward, the ICMs are not numbered.