Hello, I am beginner in Spanish and don't know if this is the proper place to insert the joke, but I'd like to ask you to explain that joke to me, because I can't understand it.

Dos viejecitas merendando.
- Eta leche no eta buena.
- Y mañana navida.

I am just guessing it could be a word game, I also understood Esta leche no esta buena. Y maňana Navidad. There must be some words replaced but I can't get it. Thank you.
When somobody says
-Esta noche es noche buena...
we respond
-Y mañana es navidad

Today is Christmas eve and tomorrow is Christmas day.

I think there is even a song that goes like that.

oh, thank you for the very interesting answer!