An official DoD article notes the fighter pilot tradition of Mustache March:

It happens every March like clockwork—men from all across the Air Force, who may otherwise normally be clean-shaven, grow out their cookie dusters, much to the amusement of many around them.

As noted here, the tradition is linked to BrigGen Robin Olds,

Olds was frequently at odds with senior leaders, and had his ways of defying the establishment—most famously (or infamously), the decidedly out-of-regs, heavily waxed handlebar mustache he sported in Vietnam.

Thus, “maverick” fighter pilots continue the “rebellion” in a tradition every year:

This is thought to have given rise to the Mustache March tradition, where Air Force men stand in symbolic solidarity to good-naturedly “protest” Air Force facial hair regulations, which state that, “Mustaches will not extend downward beyond the lip-line of the upper lip or extend sideways beyond a vertical line drawn upward from the corner of the mouth.” (From Air Force Instruction 36-2903.)

The article fails to note Mustache March immediately follows Flattop February.