Today is a rainy day. In your first sentence, either rainy or raining could fit, depending on what you actually want to say; because it is raining indicates that water is physically falling from the sky Ò€¦

It's raining (now) yesterday it was raining all day. Yesterday it rained (at least once) to talk about the type of weather you might use rainy. It is rainy Ò€¦ The reason is that in the first sentence, today is rainy, today is the object being described directly, so you don't need the pronoun 'it'. In the second however, there is a comma so after the comma, the 'it' Ò€¦ Is it correct to say today is rainy (or tomorrow will be frost. )? Normally i mention the time-expression in the end of the sentences as i was taught in past.

In the second however, there is a comma so after the comma, the 'it' Ò€¦ Is it correct to say today is rainy (or tomorrow will be frost. )? Normally i mention the time-expression in the end of the sentences as i was taught in past. It is rainy today. It will Ò€¦