Lights at the Keswick Theatre in Glenside will wink out. Students at Temple University are planning something, too, according to the Earth Hour website.
Diehard enviros, especially those with children that might get a kick out of the idea, often observe the hour, pulling out the candles, or the bourbon, or just spending time listening to the spring bird songs.
The event is in its eighth year, and every now and then a bar will offer a special Earth Hour drink.
Other than that, local enthusiasm has been rather dim.
In fact, the historic Eastern State Penitentiary, which has participated in the past, this year simply forgot, said senior vice president Sean Kelley. In a lucky coincidence, however, the lights are being repaired and will be off anyway.
Elsewhere - in more than 7,000 cities and 150 countries - the response has been more avid.
This year, the Tokyo Tower, the Wat Arun Buddhist temple in Bangkok, the Burj Khalifa, the world's tallest building, in Dubai, and the Empire State Building will join in.
In Russia, the Kremlin and Red Square will switch off; cosmonaut Mikhail Tyurin has recorded a message from the International Space Station urging protection for a beautiful, fragile planet.
It all started in Australia, where this year an array of candles on the lawn of Parliament House in Canberra will spell out "It's Lights Out for the Reef." A documentary about the impact of climate change on the Great Barrier Reef will be aired.
Indeed, in many places, the event has evolved into not just lights-out, but action-on. In Colombia, the focus is on the Amazon; in Finland, the Arctic.
And Earth Hour this year added to its website ( www.earthhour.org) a digital crowdfunding platform, Earth Hour Blue, so people can donate to various efforts.
Then again, skeptics might want to take part in a flip-side event.
The Competitive Enterprise Institute, a conservative nonprofit, is urging participation in what it's calling "Human Achievement Hour," to advance the idea that technology and innovation can solve environmental problems.
Its website urges: "Leave your lights on to express your appreciation for the inventions and innovations that make today the best time to be alive."