Monthly UFO Sightings Histogram
Here, UFO sightings are divided into months, for all years combined. Clearly, the last half of the year outweighs the first 6 months. This page was last updated December, 2005.July leads with 12% of total UFO sightings listed, mainly due to waves in 1947 and 1952. October is heavily affected by the French wave of 1954 and another North American wave in 1973.
Data is for all countries, where a reasonably accurate date is known. For a finer breakdown of all years combined, see this Daily Histogram of UFO sightings.
With fine Spring weather, one might expect the busy season to start earlier. Instead, May and June are two of the slowest months.