Basically this Feast is like Holy (Good) Friday which is a very strict fast and the only day of the year on which the Divine Liturgy in not celebrated. That is why even if the Feast of the Cross falls on a Sunday, which can never be a strict fast day because it is the day of the Resurrection, it is still a fast day. It is not by chance that it occurs 40 days after the Transfiguration (6 August) Christ was transfigured 40 days before His Passion to reveal his Glory (divinity) to His disciples and show that His upcoming suffering was voluntary. We don't celebrate the Transfiguration 40 days before Holy Friday because it would fall during lent and the joyous nature of the feast doesn't suit the penitential nature of Great Lent. That is why it is celebrated 40 days before the "other Holy Friday", the feast of the Exaltation of the Cross.
It's a bit clearer now ... 
