A blue moon refers to a full moon that occurs for the second time in a single month. According to NASA, the occurrence of a blue moon happens only about once every two and half years. This year in October, the harvest moon occurs on the first day of the month, allowing for the blue moon on the 31st.
The October blue moon will be the first to happen in the Americas since 2018. However, Farmers' Almanac states that this will be the first Halloween blue moon to occur for all time zones since 1944. The last time an October blue occurred in parts of the U.S. was in 2001. The phrase "once in a blue moon" refers to something that is a rare occurrence.