Rainy isn't the same as raining. Rainy as an adjective, indicates such as the 'rainy season' - which isn't continuous rain. Raining is what is happening - 'it's raining', or 'it was raining an hour ago', for example. The two aren't synonymous.
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 right now, while "because it is rainy" indicates that it is the sort of day where rain is extremely likely to happen, but doesn't necessarily mean that rain is ...
word usage - "It is raining" or "it is rainy"? - English Language ...
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' pronoun is needed to make the sentence correct (hence the 'it's').
is it correct to say "today is rainy" or it is "today, it's rainy"?
"I don't like it when it is rainy." VS "I don't like it raining."
Therefore, on a rainy day, we might say, "Today is rainy," or, on a cloudy day, we might say, "Today is cloudy," but if today happens to be Monday, we would not say, "Monday is rainy." Days of the week can be used with weather, but that's usually done when either (a) predicting the weather in the future, or (b) commenting on the weather in the ...
word usage - Is it wrong to say "Monday is rainy"? - English Language ...