Matthew 6:5,6 says "And when you pray, do not be like the hypocrites, for they love to pray standing in the synagogues and on the street corners to be seen by men. I tell you the truth, they have received their reward in full. But when you pray, go into your room, close the door and pray to your Father, who is unseen. Then your Father, who sees what is done in secret, will reward you."
The National Day of Prayer event isn't just inappropriate because this is a secular society; it's also un-biblical.
That is all.