Elfins are in the family Lycaenidae, along with coppers, hairstreaks, and blues. I think of them as specialized hairstreaks. In fact, they share the genus Callophrys with the green hairstreaks like Juniper Hairstreak. Elfins fly in early spring; in New Jersey this means flight periods ranging from late March through early June.
According to Gochfeld and Burger's Butterflies of New Jersey, Eastern Pine Elfin is common in the Pine Barrens in April and May. The Phillips Preserve is one of the northernmost remnants of Pine Barrens habitat in New Jersey, so it was in appropriate habitat. After mating, female elfins lay their eggs on the new growth of coniferous trees, especially Scrub and Pitch Pine.