Preview:
var days = Enum.GetValues(typeof(DayOfWeek)).Cast<DayOfWeek>();
// => [Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday]

IEnumerable<DayOfWeek> daysStartingWith(string pattern)
	=> days.Where(d => d.ToString().StartsWith(pattern));

daysStartingWith("S") // => [Sunday, Saturday]
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