This national holiday in Canada has its roots in Europe more than two thousand years ago. The Celts [Keltz] had eight main feast days. The major ones were the midpoints between the solstices and equinoxes. The minor feast days were the days of the solstices and equinoxes.
The longest day of the year was the summer solstice around June 24. It marked the mid-summer day, a time when the sun was at its most powerful. This was the day when evil spirits and fairies could cross between their world and ours, as Shakespeare immortalized in his play, A Midsummer Night's Dream.
Celts always counted their days starting at sundown rather than at midnight as we do. That meant that the 24th started at sundown on the 23rd of each month and lasted until the next sundown, when a new day started.
Herbs were thought to be at their most potent on this day. The power of the sun was strongest on this longest day of the year.