WASHINGTON — When talking about something rare or unusual, people might say it happens “once in a blue moon.”  A walk in the moonlight Friday might inspire a person to appreciate the expression.

There’ll be a full moon Friday, the second one this month. Whenever there are two full moons in the same calendar month, which occurs about once every 2 1/2 years, the second one is called a blue moon.

But Friday’s blue moon won’t look the color. It will appear as it usually does — pale gray and white, like any other full moon.

That’s not to say that the moon can never look blue. Scientists say volcanic eruptions often kick up atmospheric dust that can make the moon look blue; the phenomenon also can happen following forest fires.

To welcome Friday’s rare event, rangers at Shenandoah National Park will lead a hike to the park’s highest peak, Hawksbill Mountain.

The two-mile hike, starting at 8 p.m. on Friday, is expected to last about 2 1/2 hours. and will be lit only by the light of the blue moon.