The animal lover received hundreds of birthday wishes, which was a huge surprise for the very private actress.
"My 90th couldn't have been better," she tells PEOPLE exclusively. "I was flattered by the outpouring of love and especially by how many people generously donated to the animals at DDAF.org.
She says it was one of the biggest days ever for her animal charity, the Doris Day Animal Foundation.
"I thank everyone for their support of all the four-leggers," she says. "Tonight is the big celebration and I can't wait to see who turns up. Even at 90 years young I'm still feeling the thrill of it all!"
Told she's a national treasure, Day said to PEOPLE: "It's a funny feeling because it's not expected. I'm not trying for that but when things like that happen, it's such a surprise. It's delightful. So delightful it makes me cry. And it makes me smile. It really makes me feel good."