Also called the Chieftain’s Plaid or Laird’s Plaid
A few brief details:
The Day Plaid is made of double width fabric (approx. 142-152cm wide and 3.5-4m long / 56-60 inches by 4-5 yards), fringed at both ends. The tartan used in the Plaid should usually be the same tartan as the Kilt being worn.
It has been quoted that "the plaid should be big enough for a man and his beloved to sit comfortably and have a picnic".
Some information on the Day Plaid, including how to fold it, can be found in The Book of the Club of True Highlanders; this Club was an early form of benevolent 'Caledonian Society' founded in London circa 1817 by Donald Spalding and others, and the book forms “a record of the dress, arms, customs, arts and science of the Highlanders, compiled from printed and MS records…”
Here is a screenshot from the book, describing the Day Plaid and how to fold it:
My translation of the first section of this:
Fold the plaid twice lengthways (so it is 4 times less in width than when spread out). Then fold once crossways, lay over left shoulder with folded length side on the outer side of the shoulder and ends hanging down in front to about the waist. If you wish, you can then unfold the top section back over your left shoulder, across your back and under your right arm, then up across your chest and over your left shoulder.
Additional details follow on how to partially unfold the plaid and wrap it more completely around the body to form “a most perfect protection from the roughness of the weather”.