In the Armenian Church there are two days when he is commemorated. In 2009, they are January 24, where he is commemorated along with St. Athanasius and St. Gregory the Theologian, and October 24, where he is one of 12 theologians being commemorated. The dates will change slightly from year to year:http://www.orthodoxchristianity.net/forum/index.php/topic,16853.0.html
The rule for the first date may be something like "The third Saturday after Nativity (Jan. 6,)" and the rule for the second date may be something like "The Saturday before the Discovery of the Holy Cross," or "The fourth Saturday after the Cross of Varak," or "The sixth Saturday after the Exaltation of the Holy Cross," or something like that. It's probably the last one. I've always wished there was a book available for lay persons which explained how each saint day was calculated. Maybe some day a book like that will be available.