Thank you ALL for your beautiful posts to this thread! Before tonight, I had never heard of this feast (or perhaps, I wasn't paying attention). The pastor explained that it is one of the most ancient feast days in the church calendar, dating back to the consecration of the Basilica of Constantinople. It wasn't until I began to research it online that I fully appreciated that statement. To find links to the Syriac, Melchite and Greek rites was truly fascinating.
I noticed too, in the history of this feast that ...
When the Essenes (the Dead Sea Scrolls people) received converts into their community, they baptized them and then signed them on their foreheads with a TAU, in token that they were part of the faithful remnant who mourned the sins of Israel, and that they would be spared in the day of God's wrath.
St. Francis (of Assisi) was very fond of the passage in the prophet Ezekiel (9:4) which refers to the faithful of God all being signed on the forehead with the letter 'tau'. Francis often signed his letters with this symbol.
Pope Innocent III used this image from the prophet Ezekiel for the theme of the opening homily of the Fourth Lateran Council (1215). The Tau became a symbol from the Council for spiritual renewal in the Church. It is thought that Francis was present at this council and used the Tau from that moment on.
I have one that I brought back from Assisi 18 years ago. Such beautiful symbolism in this feast!