You have to say "It's a warm spring day". "A warm spring's day" is incorrect.

Hi Heidi! This is a great question. You can say "It's a warm pring day". If you need more help with the Saxon Genitive or other grammar topics, please visit my teacher profile and explore my lesson options. I would be more than happy to help you!

You might see terms like "a summer's day" or "a winter's day" in older literature or poetry, but these terms are almost never used anymore (even in literature). We always say "a summer day," "a spring day," etc.

It’s a warm spring day.