punctuation - When is a clause not essential?

I am having trouble figuring out when to use commas to set off "nonessential" information. Sometimes it's obvious:

Bob, who is thirty years old, is an alcoholic.

But other times I'm not sure:

The day he quits drinking * he will start a llama farm

He has his heart set on owning El Duderino ranch * in New Mexico.

In the first case, my ear says that there should be a comma at *, even though information before it seems essential to me. In the second case, the stuff after * is not essential, and yet it seems a little much to use a comma.

Even that last sentence I wrote confuses me. "In the second case" seems essential but I used a comma. Is this correct?

Does it depend on personal style and the length of the clause? Or perhaps I'm misinterpreting the meaning of "essential" in this context?

Answer

• In the second case, the stuff after [asterisk] is not essential, and yet it seems a little much to use a comma. Even that last sentence I wrote confuses me. "In the second case" seems essential but I used a comma. Is this correct?

According to the Chicago Manual of Style it's optional, even though according to them skipping the comma might be a good idea. I'd use it.

It does depend on personal style, as some authors prefer a breezier style, with less punctuation. Read Cormac McCarthy.