During the early seventh century, the Persians attacked Jerusalem, burned the Church of the Holy Sepulcher, and stole the Holy Cross, bringing the Cross to Persia. The Byzantine army attacked the Persians to get it back. The Armenians joined the Byzantines in this endeavor, and it is my understanding that the Armenian cavalry played a very significant role in retrieving the Cross.
The Cross was then brought to Constantinople. On the way there, it travelled through Armenia, being raised up and exalted wherever it went. This was a very joyful and momentous occasion in the history of the Armenian people, and this is what is commemorated every year in the middle of September.
It is celebrated with "antasdan" prayers, in which the four corners of the earth are blessed. A cross is decorated with sweet basil, which is blessed and distributed to the faithful after the service.
If you missed it, don't worry. You can celebrate it next September, and every September to come for the rest of your life.
For today, just say a prayer thanking God for taking one of the cruelest instruments of torture, the cross, and turning it into an instrument of life and joy. That is what celebrating the Cross is really all about anyway.
As Hiwot said, there are a couple of more feasts of the Cross to come in the near future. At the end of September is the Feast of the Holy Cross of Varak, and on October 28, it will be the Feast of the Discovery of the Holy Cross.