Swiss National Day is celebrated on August 1. This is also the date of an important Celtic festival, Lugh or Lughnasad.

Lughnasad goes back to the god Lugus. Lugus comes from the root word lug- – from Wikipedia: 

“A derivation from the Indo-European root leugʰ- / lugʰ- (”oath, swear“) is just as possible as an interpretation as ”bright, radiant“ based on the Welsh lleu (”light,“ compare ancient Greek λευκός- ”bright, clear, white“).”

The national holiday is celebrated at the beginning of August because the most famous federal charter, from 1291, was signed then.

From a Celtic perspective, it makes a lot of sense to sign a treaty on the day that is under the jurisdiction of the god of oaths and swearing.