According to my dictionary, the word February originates directly from Middle English "Feverer" from Old French "Feverier" yet the Modern English word more closely resembles the original Latin spelling "februarius." So where did the "v" in the Middle English and Old French come from, and why was it discarded in our current pronunciation? Also, when and why was the "r" after "b" rendered silent?
Etymonline gives information about what is known about the etymology of February. http://www.etymonline.com/index.php?allowed_in_frame=0&search=february&searchmode=none
As to the change from b to v there is nothing astonishing about this. The articulation of b and v is very similar. When pronouncing b you produce a stop of the air stream by closing both lips. When producing v you weaken this closure. Often sounds are weakened to reduce articulation work.
If in spelling the b was introduced again we may assume that is was the aim to latinize the spelling again and to show the connection with the Latin word februarius mensis.