The Transfiguration of the Lord happened in February and not in August as it is now celebrated in our Orthodox Church. The Church of Christ transferred this feast from February to August because, without transferring it to the other month, the majestic feast of the Transfiguration of the Lord would occur during the days of the Holy Forty Day Fast. That would not be congruent with the Great Lenten services, the sadness of the fast and of repentance which represents in itself the present multi-calamitous life; whereas the feast of the Transfiguration of the Lord will presage the future age in itself. The Transfiguration of the Lord is celebrated on the sixth of August instead of another time because the Exaltation of the Cross of the Lord is celebrated on September 14 in which the commemoration and memory of the Passion of Christ is again made. And as the Transfiguration was 40 days prior to the crucifixion of the Savior, the Holy Fathers, having counted forty days back from the Feast of the Exaltation of the Cross of the Lord, established that Transfiguration would be celebrated on August 6, because from August 6 to September 14 is exactly 40 days.
Source: Rukovodstvo dlia Selskikh Pastyrei [Manual for Rural Pastors] 1893.