The saying "once in a blue moon" has nothing to do with color. It refers to the relatively rare occurrence of what is called a "blue moon". (However, an actual blue-colored moon could occur from atmospheric effects such as a big forest fire on Earth causing a lot of haze. This has happened in the past!)
The real definition of a Blue Moon is simple: it is a full moon that rises twice in one month. Due to our calendar days, sometimes the days of the month are less than the cycle of the moon, which is 29.5 days. This can cause the moon to rise twice in one month, near the first and the last days of the same month. It is said that blue moons happen every 33 months or about every 3 years.
A Blue Moon cannot happen in February because the calendar month never has enough days. Furthermore, because the time of a blue moon is not fixed in terms of calendar dates, it can jump around quite a bit. During 1993 a blue moon occurred in either in August or September. If you lived east of the line that runs through the Atlantic Ocean, the blue moon occurred in September, but west of that line it occurred in August.
Upcoming Blue Moon Dates:
2007 May
2009 December
2012 August
2015 July
2018 January & March
2020 October
