Can I say
Today I am fasting/ I will fast/ I will be fasting/ I am going to fast
Tomorrow I am fasting/ I will fast/ I will be fasting/ I am going to fast
NOT A TEACHER
(1) I
think they are all correct, depending on the context (situation).
Tom: Would you like a big slice of chocolate cake with three scoops of delicious ice cream?
Mona: No thanks. I am fasting today./ I am going to fast today.
***
Alice: Would you like to join me for a delicious dinner later today?
Joe: I will be fasting later today, but thanks anyway.
***
Ramon: Excuse me for saying this, but I think that you have been eating a little too much food lately.
The Parser: Your're right! I have just made a decision: I will fast today. No food at all.
Ramon: Good idea!
(2) George: I want to invite you to my big pizza party tomorrow.
Patricia: What's wrong with you? You know that I am fasting tomorrow. I told you about that yesterday./ No, thanks, George. I will be fasting tomorrow/ I am going to fast tomorrow. But thanks loads, anyway.
***
Wife: What are you doing eating all that food? You promised me that you were going to go on a strict diet. Look at that big stomach of yours!
Husband: Ok! Ok! Stop nagging me! I will fast tomorrow. But today I want a delicious dinner.
Wife: Go cook it yourself!