We start Friday with Arbor Day in the U.S.

Watch It's Arbor Day, Charlie Brown on
Friday to celebrate the holiday

Arbor Day is celebrated on the last Friday in April (this year, the last Friday arrives tomorrow, April 26).

Conceived by J. Sterling Morton in 1872, Arbor Day is a day to plant (and care for) trees.

You can think of Arbor Day as sort of Earth Day's great-grandpappy.

Friday will also be National Pretzel Day (don't get bent out of shape) and Hug an Australian Day. Australian plumbers have it made this week (you'll recall that today is Hug a Plumber Day).

Marcus Aurelius was born on April 26, 121. Carol Burnett was born on April 26, 1933. (Happy 80th; we're so glad we've had this time together, Miss Burnett.)

The Chernobyl disaster began on April 26, 1986. Usual Suspect History.com says that, "In the explosion and ensuing fire, more than 50 tons of radioactive material were released into the atmosphere." This was an entirely avoidable event: The world's worst nuclear disaster began as a poorly-thought-out electrical-engineering experiment by less-than-optimally-trained plant technicians. The entire Ukrainian city of Pripyat was lost to the disaster (the whole population had to be evacuated). The worst of the fallout, however, seems to have rained down on neighboring Belarus. There, the Chernobyl disaster is recalled each year in a somber Chernobyl Commemoration Day. A huge exclusion zone still surrounds the Chernobyl site, and will for many years to come. Although the old Soviet Union acknowledged no more than 31 or 32 fatalities in the disaster, other estimates put the death toll at over 5,000... and continuing.