American sport is organized around a weekly rhythm that looks natural and was legislated. Restrictions on Sunday activity determined when games could be played, and the habits survived the rules.
Sunday restrictions were widespread and local
Many jurisdictions prohibited commercial entertainment on Sundays, including charging admission to sporting events, with enforcement varying by city and state.
Teams responded by scheduling around the restriction, playing on Saturdays and holidays, or by finding legal workarounds such as taking donations rather than selling tickets.
Because the rules were local, a league could face different constraints in each city, which made scheduling a legal exercise as much as a sporting one.
The restrictions pushed sport toward Saturday
Saturday afternoon became the default slot for college and school sport, and the association hardened into tradition well before the legal position changed.
That timing suited a working week that ended at midday Saturday for many people, giving a large audience free time at a predictable hour.
Once schools, colleges and communities had built their calendars around it, the day acquired an authority that no later change could easily displace.
Professional sport took the days that were left
When Sunday play became legal, professional leagues moved into it, partly because Saturday was already occupied by the college game.
An informal division emerged, with amateur and college sport holding Saturday and professional sport holding Sunday, reinforced later by broadcast agreements.
The arrangement persists even where the original legal and commercial reasons no longer apply, which is a common pattern in sporting calendars.
Evening play depended on lighting
Weekday games required artificial light, which arrived gradually and expensively, and lighting a venue changed which audiences could attend.
Evening scheduling opened sport to people working ordinary daytime hours, broadening crowds well beyond those who could take an afternoon off.
Broadcast then reorganized the whole week again, since the value of a slot came to depend on viewing habits rather than on attendance.
The inherited calendar constrains everything
New leagues and new sports must schedule around the days already claimed, which is one of the largest obstacles facing any sport trying to establish itself.
That is why emerging sports appear in summer, on weeknights or in gaps between established seasons rather than competing directly.
A calendar shaped by nineteenth-century law still determines where a twenty-first-century league can find an audience.

