"I'll pray for you."
It's the ultimate cop out.
"I'm going to do nothing to help you, but saying it outright would make me feel like crap. So you know what I'm going to do? I'm going to pretend to talk to an invisible magic sky-daddy so I can feel like I'm doing something. And after you deal with your problems all on your own, I get to claim credit! Isn't life grand?"
This is stupid even by twisted theo-logic - if the invisible magic sky-daddy really knows everything, and has the power to change things, he left things the way they are for a reason. What are you praying for? Are you smarter than him or not? If his amazing and mysterious plans are so amazing and mysterious, you better stop fucking with the grand plan right now, bozo.
Or are you appealing to his compassion? If he so lacks compassion that he won't lift a finger to help until you pray, he's a protection-racketeer. "An offer you can't refuse". Godfather indeed.
Atheists don't have the luxury of the cop-out. Atheists either help someone or they have to deal with the fact that they didn't help. Guess what - a lot of atheists do help people. To be sure, some of us don't, but we don't go around pretending we did, and we don't go around claiming credit if things turned out okay anyway.
Prayer is a great de-motivator. "I've prayed, it's in God's hand's now."
Now there's a call for inaction!
Prayer, in some circumstances, causes harm. Prayer can make people feel miserable and inadequate - if it fails, it must be your fault, and if you are brave and strong in the face of terrible odds - well you don't get to claim credit, sorry.
Atheists know there's nobody but us, and if we want things to be better, we have to do something.
National day of prayer? What a fucked-in-the-head idea.