亲爱的中国朋友们
It's a rainy day和It's raining today有区别

今天下雨 （It's raining

today）
今天一整天都有雨 （It's a rainy day雨天）
>>> 强调整天下雨
cloudy day/rainy day/snowy day/sunny day 都强调整天的天气

I've seen some Chinese people write on their post that it's a rainy day, but it's only 11 am or it's 2 pm and it just started raining. That's not what a rainy day is.