There is something strange, and perhaps even incoherent, about praying for Christopher Hitchens if the prayers are not for his recovery or for his courageous acceptance of death, but for conversion or a change of heart. Let's think about it.
I do not play the lottery; I have good reasons for not playing it; I have no desire to win it, and I believe that I would be worse off if I were to win it. Suppose you know these facts about me, but say to me nonetheless, "I am praying that you win the lottery," or "I hope you win the lottery." Surely there is something strange about praying or hoping that I get something that I don't want and that I believe would make me worse off were I to get it. But beyond strange, it may even be incoherent. Given that I do not play the lottery, there is no way I can win it; so if you hope or pray that I win it, then you are hoping or praying for the impossible. Of course, you could hope or pray that I start playing.
Hitch does not want salvation of his soul via divine agency, and he has reasons that seem good to him for denying that there is such a thing. And he presumably believes (though I am speculating here) that survival of bodily death and entry into the divine milieu would not be desirable. For one thing, his brilliance would be outshone by a greater Brilliance which would be unbearable for someone with the pride of Lucifer, the pride of the light bearer. It may also be that he believes, as many atheists and mortalists do, that the meaning of life here below, far from requiring a protraction into an afterlife, is positively inconsistent with such an extension. "How boring and meaningless eternity would be, especially without booze and cigarettes and (sexual intercourse with) women!"
Hitch has lived his life as if God and the soul are childish fictions. As a result, he has done none of the things that might earn him him immortality and fellowship with God, even assuming he wanted them. This suggests that it is not just strange, but incoherent to pray for Hitch's metanoia. For that would be like praying that he win the lottery without playing, without doing the things necessary to win it.
If a merciful God exists, then he should do the merciful thing and simply give Hitch what he wants and expects, namely annihilation. Either that, or assign him another go-round, or series of go-rounds, on the wheel of samsara until such time as he is ready to accept the divine offer of everlasting life.
As for the prayer day in his honor, Hitch won't be attending.