"Simply put, atheism is a lack of belief in any gods.
Atheism is sometimes mistakenly described as a belief that there is no god. While this definition does accurately define some atheists, it's not an accurate depiction of every non-believer. Instead, atheism is simply a lack of belief in gods without necessarily claiming certainty on God’s nonexistence. The distinction between these concepts is subtle but important.
The Issue of Belief
Disbelieving in something does not mean that you believe it is false; it simply means that you don't believe it is true. For example, someone might make the claim that Ford is the best car company in the world. You may not believe that to be true, but you may also have no compelling reason to believe it isn't true.
This is essentially the definition of agnosticism. Agnostic atheists do not believe in deities, but they also do not believe in the absence of deities. Gnostic atheists, meanwhile, do make claims that gods do not or cannot exist. Both are valid types of atheism.
Is atheism just another religion?
Atheism is not a religion. There are no holy atheist scriptures, no atheist Pope and no atheist rituals, tenets, creeds, code, authority or ceremonies. Atheists come from every possible background, race and gender. They hold a variety of beliefs and convictions, and no two atheists will have exactly the same views or ideals other than their lack of belief in any gods. Atheists may disagree on many issues.
For more information about atheism see:
• Why There Is No God: Quick Responses to 10 Common Theist Arguments
• 10 Common Misconceptions About Atheists
• Atheism vs. Agnosticism: What is the difference?
• What is God? An Objective Overview of Different Views and Ideologies
• Atheism by Country
• Discrimination against atheists
• What is the Purpose of Life?
• Where does morality come from?"

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The definition of atheism notwithstanding (above), I’m going to have to point out that it is very much a religion in the hands of men who would wield it as a doctrine, which is to say pretty much every atheist who is actively involved in waging efforts no differently that religions do.

So, what days are you going to designate as atheist holidays?

To be an atheist is to understand that no action is necessary to espouse a disbelief in a god. It’s a declaration that begs no further thought. Once declared, the subject is dropped entirely from the person’s thoughts. That’s the purpose of making the declaration: To be free of theism. It is not a call to arms to promote atheism, which is what it is in practice.

I see much ado about atheism (from atheists) when I should be reading nothing of it from them. If I do read the gobs and gobs of silliness dedicated to atheism and sort through the pap of it all, I find religion at its heart. It has become a doctrine.

The arguments theists grapple with in defense of who they are should never have to occur. But, atheists are the antagonists of ill intent who call them out to argue, write books against their views and generally disrespect them at every turn. This is simply true out of the knowledge that there is no useful purpose to a true atheist to oppose theist views. It's strictly morose entertainment.

When this kind of atheist behavior gains popularity and public support from atheists at large, a religion is born and in this example appears a sad display of man's desire to hurt for no logical gain or useful reward, and in direct opposition of its definition.

Warn yourselves that you who persist in waving banners are (sadly) no different than those you seek to hurt.