It was a tale of two classics, a tale of two eras, for Green Day at Citi Field on Monday night. Like a dog-days double-header — in which the Mets home field was transformed into a melting mosh pit by the heat and humidity — this night was set in two acts as the punk-rock triumvirate of singer-guitarist Billie Joe Armstrong , bassist Mike Dirnt and drummer Tre Cool celebrated two anniversaries: the 30th anniversary of their major-label debut, 1994’s diamond-selling “Dookie,” and 20 years of “American Idiot,” their 2004 concept-album classic that was adapted into a Broadway musical. But in actuality, this was the “Saviors” Tour — named after the Rock & Roll Hall of Fame trio’s 14th studio LP that was released in January — which is hitting US stadiums through Sept.

28. And after the Linda Lindas, Rancid and Smashing Pumpkins had already played in what amounted to a mini Lollapalooza the day after the festival finished in Chicago, Green Day came pogoing on the stage in all their spiky-haired glory to “The American Dream Is Killing Me,” the stomping “Saviors” opener. In this critical political moment, suddenly — 30 years after an unserious title like “Dookie” was hardly a harbinger for longevity and enduring importance — Green Day was just as relevant as ever.

Then the threesome quickly pivoted back three decades — and gallons of hair dye ago — to “Dookie,” which they played its entirety. And apparently Armstrong wanted cell-phone cu.