"to be sulk for today" sounds quite strange, but the word 'sulk' is like 'refunfuñar'. It's what happens when someone is upset and then they passive-aggressively go quiet and refuse to talk / participate in an activity.

I found the dialog online: A: I came as soon as I could. How are you doing? B: That depends. Did you get me ice cream sandwiches? A: Of course I did. This is not my first rodeo, you know. B: Thank you for coming. I can’t believe Derek cheated on me. A: I know. But you’re better off without him. You’re way out of his league. B: You think so? A: I know so! Now, here’s the plan. We stuff our faces, watch some movies and sulk for today. And then tomorrow you shake it off and get back out there. You deserve better. I agree with everything other people have written, that "sulk for today" doesn't sound correct. In the dialog above, all the other sentences sound fine, but I'd change the last line to the following: A: I know so! Now, here’s the plan. Today we stuff our faces, watch some movies, and sulk. And then tomorrow you shake it off and get back out there. You deserve better. I think that's more natural English. And in that case, "sulk" just means basically "to not feel like talking because you're upset". Looking through that textbook, I'd say it's pretty good at teaching expressions, but there are some mistakes here and there, like on the next page, it says "She will call as soons as she can." That should be, "She will call as soon as she can." ¡Suerte!

Well "sulking" means being in a bad mood, like being depressed about something, but I don't think that sentence really makes sense to be honest

Hey Wilson I believe some more context is needed for that phrase since it doesn’t make much sense! “Sulking” is being annoyed at something and not want to discuss it. Other than that I’m not sure what else I could get from that 😂

It means to be angry about something and refuse to talk about it.