It happens once every two or three years, or, to be more precise, seven times every 19 years.
A blue moon.
That's when, according to modern reckoning, you have two full moons in the same month.
That will happen Friday evening, and the Friends of the Monarch Trail invite you to celebrate the celestial event with an earthly one, a Blue Moon Party beginning at 5:30 p.m. on the County Grounds.
The party will feature kite flying (wind permitting) and Irish music presented by Ceol Cairde. Other activities may include scientific tagging of any early migrating monarch butterflies that may be wafting through the County Grounds.
Guests are encouraged and permited to park at the Milwaukee County Parks Department Headquarters Building lot, 9480 Watertown Plank Road.
At 6:15, there will be a guided tour of the Monarch Trail, including the monarch migration habitat zone surrounding the Eschweiler Buildings.
The Monarch Trail was founded to call attention to the natural phenomenon of dozens to thousands of monarchs roosting in trees on the high ground near the Eschweiler Buildings every autumn, as the colorful insects make their way to central Mexico for the winter.
Moonrise is at precisely 7:13 p.m., and this blue moon may be a little more special than most, rare as they are.
"You should be able to see the moon rising as the sun is setting," said Barb Agnew, the founder of the Monarch Trail.
That's true elsewhere, too, but one advantage of seeing it from the County Grounds near the Eschweilers is that it is among the highest points in Milwaukee County.
Agnew especially encourages families to bring children out for the event, if only to get them outside.
"There aren't any rules, even about staying on the path or touching this or that," Agnew said. "It's a great chance for children to just get out and romp around."