Don't take this the wrong way. It is fine to observe Earth Day. Just don't limit that to a commemoration.
Don't think of it the same way some people view Valentine's Day, which too often has become a romantic day of obligation.
A better approach would be to celebrate it similar to the way some people mark the new year. Resolve to be a better steward of this planet. That could mean doing - or not doing - something on a small scale.
For example, stop using bottled water; reuse existing containers instead. Start a compost pile. Stop using disposable grocery bags. Start washing your laundry in cold water. Donate unused items to a thrift store or the Humane Society garage sale this week, Shop at thrift stores and garage sales.
Today is Earth Day, but you can celebrate it every day on Earth.