My code before and after reading the docs for Ruby's Date class from stdlib.

date.rb

# before
class Date
  def first_business_day_of_month
    date = beginning_of_month
    loop do
      return date if date.workday?
      date = date.next
    end
  end
end

# after
class Date
  def first_business_day_of_month
    beginning_of_month.step(end_of_month).detect(&:workday?)
  end
end