"O que" will usually mean WHAT. We will typically use "O que" for open questions like: O que você quer fazer hoje? = What do you want to do today? "Qual" will typically be WHICH. Or when you specify the kind of thing you're talking about. What flavor = Qual sabor (also QUE SABOR) What job = Qual profissão (also QUE PROFISSÃO) Qual sabor você quer? = Which flavor do you want?

I will try to explain it in such a way that you feel it. O que é o problema?= What is the meaning of "the problem"? Qual é o problema?=What is the problem? If you see "hoje" as a noun. O que é hoje? = What is the meaning of "today"? If you see "hoje" as an adverb. The meaning depends much on the context. Without any context most probably you would interpret "ser" as "acontecer" (you can do it). O que é hoje? = What happens today(adverb.)? In other specific contexts you could say Qual é o dia de semana, hoje? Qual é, hoje?

O que is commonly more used as “what”, while “qual” means “which”. Qual camisa? - which shirt? O que é isso? - what is this Qual(which) é o problema? Would be correct While “o que é hoje?” Would be better phrased as “que dia é hoje”(what day is today) just as “o que é hoje” would be roughly translated and interpreted as you asking what “hoje” means { what is ‘today’?}

você irá usar "O Que " somente quando houver apenas umas resposta. por exemplo O que é papaya ? "Qual" será usado quando houver mais de uma opção de resposta. por exemplo Qual destes é seu carro? Which = qual ? What = o que ?

What Portuguese are you learning from Portugal or the Brazilian? The question .... What is it today? this question is meaningless, could be What do you have for today? better saying What's good today? The second question is correct. What is the problem? I think the more you practice the more you will know how to use the words.