Both are correct!! Perhaps you are confusing the the verb "celebrate" with the noun "birthday celebration." While your "birthday" is the entire day, a "birthday celebration" is usually only part of the day, as in a "birthday party." I had a birthday celebration today. Means that: I was celebrating my birthday today. I celebrated my birthday today.

Both tenses are correct. I was celebrating my birthday today is past continuous - we use past continuous to describe an action or event in progress at a particular time in the past. I celebrated my birthday today is the past simple - use the past simple for finished actions or events in the past. For example: While I was celebrating my birthday today with my family, a friend called me on the phone. I celebrated my birthday earlier today with my friends and now I'm watching a film. Hope this is helpful :)

Both could be correct depending on the context. The neutral and most common way to say it is to use the simple past - "I celebrated my birthday today." However, if something happened during the period you were celebrating your birthday, you could use the past continuous tense: E.g., "I was celebrating my birthday today when I received news of my sister's accident." Of course, you can also use the present continuous tense, such as "I'm celebrating my 31st birthday today." If you are confused as to why it is possible to use the past tense ("celebrated") for *TODAY*, which should be in the present tense, there are at least two reasons why the past tense is justifiable. (1) You are making this statement at the end of the day, when the day is almost over. Then you can certainly say "I celebrated my birthday today." (2) You are referring to a specific event related to your birthday, such as a birthday party, that has already occurred earlier in the day. In this case, you can also say "I celebrated my birthday today." Hope this was helpful.