YOU SAID:
It's a rainy day today.
INTO JAPANESE
今日雨です。
BACK INTO ENGLISH
Today it is raining.
INTO JAPANESE
今日は雨です。
BACK INTO ENGLISH
It's raining today.
INTO JAPANESE
今日は雨です。
BACK INTO ENGLISH
It's raining today.
That didn't even make that much sense in English.