I have just learned that 2026 has been declared "The Year of Mathematics"or YoM2026, by the Conference Board on Mathematical Societies. Unlike previous celebrations noted on this blog, such as last year being the international quantum year, this YoM2026 designation seems to be restricted to the United States. At time of writing, they have announced that a bipartisan Congressional resolution has been introduced in the U.S. Senate.
Why 2026? Apparently this year the International Congress of Mathematicians will be meeting in Philadelphia, in the year of the United States' 250th anniversary. The quadrennial Congress was held in the U.S., according to Wikipedia, on only two previous occasions (1950 and 1986). Otherwise, I am not aware of any particular anniversary being celebrated in 2026 in connection with mathematics. In this respect the selection of the year seems as arbitrary as 2020-2021 as the International Year of Sound. In contrast, I noted several epochal anniversaries being celebrated when 2015 was designated the International Year of Light and Light-based Technologies. (Wikipedia tells us that in addition to the ones I noted in my post at the time, there were also 1865 and 1965, respectively, anniversaries of Maxwell's electromagnetic theory and the Penzias/Wilson cosmic microwave background discovery.) Of course 2005 was the World Year of Physics to celebrate the centenary of Einstein's annus mirabilis. Then 2011 was the International Year of Chemistry, celebrating the 100th anniversary of Marie Curie's Nobel Prize in Chemistry (um. she also won a Nobel in Physics in 1903) as well as the centenary of the International Association of Chemical Societies.
So, in contrast with these various physics-related celebration years, there is no specific anniversary being celebrated by YoM2026, nor is the celebration international. Even the International Year of Statistics (2013) was a global event, though I am not aware of any particular statistics anniversary being celebrated that year. Last year's quantum year celebrated, of course, the centenary of Heisenberg's matrix mechanics.
Perhaps the granddaddy of them all was the International Geophysical year in 1957-1957, which Wikipedia tells us traces its heritage the International Polar Years of 1882-1883 and 1932-1933. However, unlike the more recent celebrations I noted above, the geophysical and polar years featured surges of actual research and international collaboration. In comparison, the more recent celebratory "years" seem to be largely about outreach and propaganda.