With Day's victory, she became the second Black woman to ever win in the category -- which itself has only had 12 Black nominees in the award show's history.
"That's another layer of wow," Day shared with ET's Kevin Frazier, referring to being the only Black actress to ever win in the category, apart from Whoopi Goldberg for The Color Purple in 1986. "It's so exciting, and also heartbreaking at the same time. Because Black women are so dynamic and we have so many amazing, layered stories."
However, Day says she is still overwhelmed with joy for the honor, noting, "I feel nothing but gratitude."
Day's commanding and highly celebrated portrayal of Billie Holiday in the biopic The United States vs. Billie Holiday is after years of hard work and dedication to her craft. The performer said the experience and the recognition is coming at a time in her life when she was ready for it.
"I like to consider myself a deeply spiritual person, and I think the beauty of God is he does things when they reach full maturation," Day said. "There's something beautiful about the idea of full maturation and being prepared for something. Not just physically, and not just in your career, not just economically, but spiritually, emotionally and psychologically ready."
For other aspiring artists and dreamers pursuing their ambitions, Day had some words of encouragement that she's learned from her own journey.
"If it feels like it's taking a long time... do not begin to feel unworthy. Do not begin to feel inadequate, please do not quit," she shared. "There is space for everybody."
At this year's big show, Day was up against Viola Davis (Ma Rainey's Black Bottom), Vanessa Kirby (Pieces of a Woman), Frances McDormand (Nomadland) and Carey Mulligan (Promising Young Woman).