The first day is the best moment to set expectations, because everyone is present and attentive. State plainly how the class will run: how to participate, how discussion will work, how to reach you, and what respectful engagement looks like. Explaining the reason behind a policy, rather than just the rule, makes students more likely to follow it.

Where it fits, invite students into a few norms, since people honor rules they helped shape. Setting expectations on day one prevents most of the friction that otherwise shows up later, and it gives you something fair and clear to point back to if a problem arises. Clear from the start beats corrective later.