Ah, A Week! My 5th favourite holiday-that-isn't-a-day-or-holy of the year.
A Week, which I assume is short for Atheism Week, but which could just as easily be Agnosticism Week, Afairyism Week, Anunicornism Week or Abullshitism Week, skirts a little bit close to the "atheism is just a religion" line, but I think stays safely on the practical necessity side of it. The idea is for godless heathens on social networks to set aside their usual rejection of any shared, unifying identifiers, and instead all display a nice, big, clear letter A as their profile picture for one week a year. The main purpose of this is to let physically or socially isolated atheists quickly and easily see where they might find some sympathetic ears, or at least to give them some hope that they aren't totally alone in the world. It can be hard to imagine if you've never felt that sort of isolation, but any little bit of contact with the somewhat-like minded can mean a lot.
A secondary purpose (as far as I'm concerned, anyway) is to put a dent in theists' assumptions about their dominance and prevalence. It's easy to assume that everyone is like you, especially when the ones who are different specifically reject going out of their way to look different and distinct. And that assumption can feed a terrible arrogance and lack of self-questioning. Shaking that up every now and then is good for everyone.
Both those things together add towards making public atheism more socially acceptable.
Sadly, not enough atheists embrace this event. Some of the hardcore very-very atheists feel that anything that paints us as a unified group takes us one step too close to acting like a faith-based organisation. This is a mistake; all movements, even the informal ones, have to operate by procedures of some sort, and if a procedure is good and useful, most or all of the time, then why shouldn't most or even all people adopt it? That's practical and logical, not group-think faith bullshit.
On the opposite end of the scale are the softcore wishy-washy atheists, those who really don't believe in any gods, but who feel social pressure to keep this to themselves, or who don't want to be associated with the more shouty, "angry" (as they are often falsely called) hardcore atheists. This is also a mistake; if you don't publicly claim your own identity and shape it more to your liking, then you have only yourself to blame if you're judged according to the behaviour of others who share that identity, but use it differently to the way you would. Take charge for once, and carve out your own niche in the public perception of your lack of belief. Show by example how there are different ways of playing it.
And besides, think of the poor, isolated atheist kids stuck in some strict religious community! It's so hard to say whose A Week participation will be the one that reaches them, but the more people who take part, the better the chances. Surely your (perhaps perfectly valid) personal concerns are worth overlooking for just one week, in order to give them a bit of hope?
The only important thing is that you don't take religious claims seriously (even if you feel there's still some reason to take religious people seriously). If you're not convinced that there's really any supernatural magic person or people in the sky (or wherever your local faith likes to store them), then you're atheist enough, and this is as good an opportunity as any to let people know about it.