It happened in 778 at dusk. The Frankish Imperial Army had failed in the plot to conquer Zaragoza and returned home through Basque lands. For unclear reasons Charlemagne had ordered to demolish the walls of Pamplona, maybe because they did not expect to be able to control it against both Basques and Muslims, what apparently outraged the Basque People to no avail.
Some days later, a day like today 1236 years ago, they had to cross the Pyrenean passes and mostly they did without problem, but when the rearguard, made up of the elite knights of the Frankish Empire, including some of the most powerful men in the realm, marched through them, they were ambushed and massacred.
It was the only major defeat of Charlemagne ever.
|12th centenarian commemorative plate|
Later two subsequent battles in 812 and 824 consolidated Basque independence south of the Pyrenees.
Some more details at Wikipedia, for example.