Returns the corresponding human readable day name of an integer (0-6).
type DayName =
| "Sunday"
| "Monday"
| "Tuesday"
| "Wednesday"
| "Thursday"
| "Friday"
| "Saturday"
function dayName(day: 0 | 1 | 2 | 3 | 4 | 5 | 6): DayName
// Example:
dayName(3) //=> "Wednesday"