"I already was about to go to (no article /a /the) class tomorrow morning."

I was told that it is possible to say:

"I already was about to go to (no article) class tomorrow morning."

without an article after the the preposition "to" before the countable noun "class" in this example.

Well, the reason for that is not understood to me, while I obviously understand the difference between the articles "a"/"an" and "the" I don't understand the omitting of all articles here. If it's just non specific class we can say "I already was about to go to a class tomorrow", so why we omit the indefinite article here?

Answer

You go to school, university, classroom, office, and college etc for a purpose. And, this is a regular affair.

So, if the event is regular, and the purpose is clear (of studying), zero article.

Henry and Keith go to school

But when you emphasize the destination/place, you put an article.

I am going to the school to pick Henry and Keith.

Similarly,

I went to hotel soon after I landed there (for staying purpose)

but...

I went to the hotel to meet Mr Anderson (for specific meeting)

In above all examples, the place is specific and thus, the article the is put. But to answer your question, if there is a specific purpose for a specific place (like meeting someone), you may put the article or else, if it's a general event, you omit the article.

More on zero article is here.