Consistent with US law, President Barack Obama proclaimed Thursday, May 3, 2012 to be a National Day of Prayer.  He also called specifically for prayer for US troops:

Let us pray for the continued strength and safety of our service members and their families…

I invite all citizens of our Nation, as their own faith directs them, to join me in giving thanks for the many blessings we enjoy, and I call upon individuals of all faiths to pray for guidance, grace, and protection for our great Nation as we address the challenges of our time.

His proclamation for a day of prayer contained a subtle acknowledgement of those who do not pray:

We give thanks for our democracy that respects the beliefs and protects the religious freedom of all people to pray, worship, or abstain according to the dictates of their conscience.

Members of the US military will also gather for days of prayer wherever they may be around the world.  Hopefully, atheists will grant them this opportunity to exercise their faith at the invitation of their Commander in Chief, even if they are offended by it.  Of course, some atheists are attempting to organize their asterisk on the National Day of Prayer, since they can’t come up with their own idea…

With reference to the Religion Clause.