Every April since 1970, many Americans have celebrated Earth Day. While the holiday does not have the buildup of Christmas or the whoop-de-doo of the Fourth of July, it has become tradition in many places to take a hike, clean up roadsides and wrack lines, or take part in other outdoor activities. Numerous local organizations have laid on events this year, among them the South Fork Natural History Museum in Bridgehampton and the Nature Conservancy’s Mashomack Preserve on Shelter Island.