School reformers, mostly middle-class white women, invented kindergartens in the late-19th and early 20th centuries. The innovation of the “Kindergarten” was an import relying on the ideas and practices of a 19th century German reformer named Frederich Froebel. Using his ideas, civic-minded women created kindergartens in response to concerns about so many urban children left on their own daily as immigrant parents worked long hours in factories and sweatshops.
By the 1920s, many school districts had installed this innovation into their grammar school thus converting the 1-to-8 age graded school into the now familiar K-6 elementary school.
The history of kindergartens revealed tensions between play and learning basic academics such as reading, adding numbers, following directions, working in a group and treating one another well. These tensions unfolded in training kindergarten teachers how they should teach. That debate over whether kindergartners should spend more time on play or academics continues in 2023.
While many districts adopted kindergartens, in 1920 just over 10 percent of eligible five year-olds entered kindergartens. by 2020, eighty-four percent attended kindergartens. No surprise, then, that this innovation has become a familiar experience for five year-olds in the U.S.

The 1879 Grand March of Boys and Girls in New York City’s Free Kindergarten, established by the Society for Ethical Culture. Credit: Frank Leslie’s Illustrated Newspaper, Granger.

Free Kindergarten for Colored Children of New York City, 1901. Credit: Free Kindergarten Association for Colored Children, Sixth Annual Report.
From these beginnings of an innovation, throughout the 20th century, school districts added kindergartens to their elementary schools. Nearly 85 percent of five year-olds attend public kindergartens in 2020.
Current kindergarten rooms in the U.S. look like this:

Kindergarten students in Robin Bryant’s class are learning how to add and subtract (SanFrancisco Bay area school)

Kindergarten in Penn Manor School District (PA)

Paraprofessional Kristina Wilcox, left, and instructor Marissa Reitan, second from right, work with kids in a preschool class at East Ridge Elementary in Ogden (UT) on Monday, Feb. 7, 2022.

Kindergarten classroom in Albany (NY) elementary school
What does a day in kindergarten look like to both teachers and students. Following examples offer partial answers to the question. You Tube videos of kindergarten teachers in across the country follow.
I apologize in advance for the ads that accompany these videos and descriptions. As anyone who uses the Web knows, ads are part of the territory within which researchers work.
One school district describes what a typical school day should be like for kindergartners. Here is the URL since the description is too long for this post: https://www.scasd.org/Page/21
With the introduction of the innovative kindergarten nearly a century and a half ago, going to school in the U.S. in 2023 means that five year-olds enter the wider world beyond the family. And it is in these spacious rooms with colorful furniture, rugs, chairs, and learning centers (including a bathroom) that the nation’s children face the continuing tension between how much time in a day should be devoted to play vs. how much time should be devoted to learning to count, read, and time teaching children how to behave in groups.