Even the celebrations appear similar.

“They shared a meal, they had banquets, they went out, they took boats on the river; it was in some ways very close to what you see now,” Dr. Haikal said. “This is why it may relate. But I underline ‘may.’ ”

The holiday has long been observed on the Monday after Coptic Easter, a merger that most likely happened during the early centuries A.D. when many ancient Egyptian practices existed alongside Christianity.

The arrival of Islam in Egypt in 640 apparently did not affect the holiday.

Dr. Haikal said Sham el-Nessim is now a national holiday, not a religious one, and she doubted anyone could strike it from the calendar.

“Festivals are here to cheer people up, and they need something to cheer them up,” she said. “They will not give up their holidays easily.”

Nor will many give up their feseekh.

On Monday, large families with bags of food staked out patches of grass in the shade of scattered trees at a hilltop Cairo park named after the nearby Al-Azhar mosque and university, Egypt’s historic seat of Muslim scholarship.

Some said the government’s warnings and recent reports of the police confiscating bad feseekh had made them opt out. But many still indulged, the smell of their meals wafting over the footpaths.

Seated on a blanket in the shade with his wife and three daughters, Mohammed Hassan lifted a yellow-gray fish from a plate, tore it open and drizzled lime juice over its pink flesh.

He said he had heard the warnings, but only bought feseekh from people he knew and trusted.

“It’s excellent,” Mr. Hassan said, tearing bits of meat from the fish’s spine and popping them in his mouth. “There’s nothing to be afraid of.”