The Blue Moon is a rare occurrence that involves the entire moon appearing within a certain period and often refers to the full moon. However, it is used to describe how the moon actually looks and for different reasons it turns blue. Astronomers at Cambridge usually enjoy the appearance of the Blue Moon at the end of January with. A rare celestial manifestation occurs when two full moons appear in the same month the second moon is called the Blue Moon.
And according to tradition, when two full moons appear in the calendar month, the second is called the Blue Moon. The last time the blue moon appeared in the sky was July 2015.
The name can be misleading because the blue moon does not appear blue, only in rare cases the moon turns into a bluish tinge. For example, during the 1883 eruption of Krakatoa volcano, this change in color occured because of particles of dust in the atmosphere. which dispersed the red light causing the moon to reflect the blue light and shows us down on the earth as if it is blue.
Source
https://www.thaqafnafsak.com/2018/02/القمر-الأزرق-ومتي-وأين-يمكنك-رؤيته-بال.html