Unfortunately, neither response given to you by previous posters is correct. Firstly, we can most certainly use Present Perfect (have/has + past participle) with the word "today". 'I have attended a session today' (notice the article 'a'). In this case, the day is not yet complete and you might attend another session later on, though we would probably add the word 'already'. However, by using the definite article "the", you are referring to a specific session which is now complete. As it was a 1-time, completed action, even if it is still 'today', you would use the Past Simple - Today, I attended the session The second sentence could not be used as a stand alone sentence. The Past Perfect (had + past participle) is used to describe a past event that happened before another past event. As you have only mentioned 1 event (the session), there is no established timeline, so you wouldn't use the past perfect.

Today, I attended the session. You used "today" to mark when the session happened. If we know the exact time when something happened in the past, we use Past Simple, not Present or Past Perfect. I hope this helps! ☺

I attended the session today. - Past Simple, finished past action. I realized I hadn't attended session today. - Past Perfect. The action that happened BEFORE another action in the past. (1 - I hadn't attended, 2 - I realized).

Firstone

The have is optional but most people would drop it unless you wanted to stress that you did it. otherwise it is implied. - Today I attended the session - or - Today I have attended the session. (suggests the person asking knows what session you are talking about and they might ask "Was it any good? I went last year.") - Today I attended a session - or - Today I have attended a session. (suggests the person asking does not know what session you are talking about and they might ask "Which session?" - Yesterday, I had attended the session, but it was boring. (again you can drop the had to sound more natural). using had here kinda implies a but. Just saying "Yesterday, I had attended the session." the listener will be waiting for a "but..." or "and...".