Что я знаю (It's better to put here "я" because it's the title and the first time you mention yourself.

-----

сегодня  - just wrong spelling;

"я знаю, что..."; "я знаю, потому что" - we always put comma before words "что" and "потому что". But remember that between "потому" and "что" we don't have comma as it's like the one word "because".

"Я знаю, потому что я смотрю на календарь." - means that you are looking at callendar right now in this moment. If you want to say that you looked at it recently, you should use "я посмотрел на календарь".

"Я знал, что вчера воскресенье, потому что (вчера) я посмотрел на кадендарь." - here it's not very clear what you wanted to say. It's better to add some word about time. For example here I've added "вчера". Means that you knew that yesterday was sunday because yesterday you looked at callender. I think that is what you've meant because you used "Я знал" in the past tense.

"Я буду знать, что завтра вторник, потому что (завтра) я посмотрю на календарь." - Here it's also better to add "завтра" and to change "буду смотреть" to "посмотрю", because it's different meaning. The first one is like "you will be looking" (for some time), and the second is "you will look" (in general).

Hope it's what you wanted to say and I made it a bit clear for you.

Best Regards,
Alla

Что я знаю

Что я знаю

Что Знаю