"I am fasting" is correct and people with understand you, however many people do not associate Advent with fasting, so if you say "I am keeping Advent", you may have to explain what that means.

Just curious - which date(s) are you talking about here? For significant religious dates and periods, we'd say "observe". "I am observing Advent". A more accurate phrase would be "I am fasting in observation of Advent." If it's a single day or evening, I would use "keep a vigil" - but that requires a specific context. I understand this is a Russian tradition, so we would need more context to explain it in English.

I am fasting - this is grammatically correct. I understand it. What I don't understand is the reference to advent? Perhaps you mean Lent? I don't know anybody who would consider fasting during Lent but I do know a few who try to cut down on things like smoking, drinking, and eating too much! But you would have to ask someone familiar with the Christian religion. I'm English, but not Christian... so I really don't know what Lent (or Advent!) is!

You may have been thinking of Lent, which is the 40 day period before Easter, more traditionally associated with fasting in Christian countries. "Fasting" these days doesn't always have religious connotations, though it does mean some form of self-denial (denying your appetites) for a good reason. If the reason was just to lose weight, the more common word would be "diet" (noun and verb).

Yes it is a very clear communication. Remember, the correctness and the clarity of your communication is not dependent upon the ignorance or knowledge of others. Your clarity and correctness is independent of the ignorance or knowledge of others. Susan is giving you a bit of a mixed explanation