Before I forget, as of 4 years ago, when you enter Mexico, you are given an entry paper with the stamped date on it. You MUST keep this paper to present for when you leave Mexico, or you have to pay a fee.
Like a lot of stuff in Mexico and other nearby countries, things like this are pretty wishy washy or unclear , but you will get a fine if you don't hold onto this.
I really encourage you to learn some Spanish. Don't know when you are going, but it makes a difference.

Oh, you may find it cold in December or January, perhaps less so on the coast, I been usually higher up in altitude but had some really cold days in January. Nights of 5c or less and cool rainy days sometimes. Just be prepared and look up weather.