The epic passed down among the Haudenosaunee has the founding during a total eclipse, or “a darkening of the Great Spirit’s smiling face.”

The Haudenosaunee's history and traditional sources date the founding of the league to around August 31, 1142 during a total eclipse.

Academics estimates vary, but the consensus seems to be that it was founded around 1450-1550. One eclipse, which is put forward by Dean Snow, is June 28, 1451.

It's possible that there isn't an eclipse linked to the founding, but eclipses are big cosmic events that could have been interpreted to be divine, so they're useful in attempting to date the Haudenosaunee's founding.

Post-contact dates are fairly contentious, despite the introduction of some Jesuit traditions into some tellings of the the Deganawidah epic, among other issues. The epic links the Haudenosaunee's founding to a cannibal cult, but Europeans should have replaced any cannibal cult as the enemy post-contact. The omission of Europeans from the founding epic makes any such connection tenuous and difficult to maintain.

Source: Buck: Deganawida, The Peacemaker

Read an English translation of the Gibson-Goldenweir Deganawida epic here: http://www.amphilsoc.org/collections/style/pdfoutput/Mss.Rec.125-ead.pdf