World Humanist Day is younger than the international body most closely associated with promoting it: Humanists International, founded in Amsterdam in 1952 as the International Humanist and Ethical Union, predates the solstice observance by roughly three decades, and its founding Amsterdam Declaration set out a statement of humanist principles that the organisation has revised over subsequent decades as its member organisations and the countries they operate in have multiplied, giving the day a considerably older institutional home than its own comparatively recent history as a fixed annual observance.
Because Humanists International is a federation of national member organisations rather than a single worldwide congregation, the day is marked unevenly and independently from country to country, with some national humanist associations holding organised public lectures or media campaigns and others marking it more quietly or not at all, so the character and visibility of World Humanist Day in a given year depends heavily on the capacity and priorities of whichever local member organisations choose to observe it, rather than on any centrally coordinated single-day campaign.
Because the day is fixed to the June solstice, a northern hemisphere midsummer, some southern hemisphere humanist organisations have raised the same seasonal mismatch that affects other solstice- and equinox-based observances, and have on occasion marked local events around the December solstice instead, though Humanists International has not formally split the observance into hemisphere-specific dates the way some neopagan calendars have, so the June date remains the standard reference point cited internationally even where the season it falls in is winter rather than summer.