When is it appropriate to wear a fly plaid? When is it not?
To answer that, we must look at the origins and history of the fly plaid. It originated in the 1950s when jet-liners were really becoming popular for global travel. Highlanders of the time were looking for a convenient blanket to take on the plane so they could sleep on those long overseas flights. Hauling a full-size plaid onto the plane was just too much. So a smaller version was adopted for trans-oceanic flights which would provide light cover over the torso whilst sleeping, and be portable enough to make for easy stowing when the flight was over. It was designed with one corner pleated so it could be tucked into one's collar whilst eating an airline meal so as to serve as a long napkin. It was very well suited for airline travel. Hence the term "fly plaid". This is its most appropriate (and originally intended) role.

There was a curious occasion where a Highlander by the name of Hugh MacSnodswold was to be married to an American girl in New York. He was to fly from Scotland to America on the day of the wedding and proceed directly from the airport to the church. So he traveled in his finest kilt outfit for the wedding. This being a trans-Atlantic flight, he of course brought his handy fly plaid. However, the plane was late in arriving due to weather, and he was all discombobulated when he arrived. In his haste to disembark from the airplane, he forgot about his fly plaid, which fortunately got hung up on the epaulet of his jacket. As he ran through the airport, kilt pleats swishing, the fly plaid managed to find its way over his shoulder and behind him. He arrived at the ceremony in the nick of time, and the guests were delighted by the stylish addition to his otherwise traditional wardrobe. And so the tradition of the fly plaid being worn at weddings began.

And now you know "the rest of the story".