```options.keys => ["repeat", "by_day_of", "interval", "interval_unit", "day_of_month", "day_of_week", "until_date", "start_date", "monday", "tuesday", "wednesday", "thursday", "friday"]

[3] pry(#<Recurrence>)> DAY_NAMES = Date::DAYNAMES.map(&:downcase).map(&:to_sym)
(pry):37: warning: already initialized constant DAY_NAMES
=> [:sunday, :monday, :tuesday, :wednesday, :thursday, :friday, :saturday]

options.keys & DAY_NAMES
=> []
[4] pry(#<Recurrence>)> options.keys.map(&:to_sym) & DAY_NAMES
=> [:monday, :tuesday, :wednesday, :thursday, :friday]