A blood moon, which is a total lunar eclipse where the moon takes on a reddish color, typically occurs about 2 to 4 times per year. This happens when the Earth comes between the sun and the moon, casting its shadow on the moon and filtering sunlight through Earth's atmosphere, which gives the moon its red hue

. More specifically, lunar eclipses (and thus blood moons) happen during eclipse seasons, roughly every six months, when the moon's orbital plane aligns with the Earth and sun. However, total lunar eclipses (blood moons) are less common than partial eclipses and usually occur a few times each year globally

. Additionally, the term "blood moon" is sometimes used to describe a lunar tetrad, which is a series of four total lunar eclipses occurring about six months apart over two years. These tetrads are rare and happen only a few times per century

. In summary:

• Blood moons occur roughly 2 to 4 times annually.
• They happen during total lunar eclipses when the Earth’s shadow covers the moon.
• Lunar tetrads (four blood moons in a row) are rare and happen sporadically over centuries