Math geeks and pi lovers, rejoice. Pi Day this year has special significance because Saturday's date extends the symbol 3.14 (3/14) by two more mathematically correct digits 3.1415 (3/14/15).
For those who are clueless, it's a day set aside each year -- always 3/14 -- as an homage to the symbol for the ratio of a circle's circumference to its diameter. Even if you don't get it, you can still take advantage of discounts and offers in honor of pi.
The Exploratorium in San Francisco invented Pi Day, or at least started celebrating it, 27 years ago. Visitors who turn up Saturday will receive free admission to the museum and can participate in a day of Pi-themed activities.
Organizers also will be marking the birthday of Albert Einstein, who was born on March 14, 1879. Info: Exploratorium's Pi Day, Pier 15, Embarcadero at Green Street; (415) 528-4444
The Charles Hotel in Cambridge, Mass., will launch a flash sale that takes 31.4% off the hotel's best available room rates. Discounted room prices start at $178 a night.
The sale starts at 6 a.m. and ends at 2 p.m. PDT Friday. You can save money and book a stay for Saturday through May 11 -- perfect for those who may need a room for an upcoming college graduation or a spring college tour. Info: Charles Hotel, 1 Bennett St.; (800) 882-1818