class ActionView::Helpers::DateTimeSelector

def day_name(number)

day_name(1) # => "1st"

If the day_format: ->(day) { day.ordinalize } option is passed to DateTimeSelector:

day_name(1) # => "01"

If the use_two_digit_numbers: true option is passed to DateTimeSelector:

day_name(1) # => 1

Looks up day names by number. 
def day_name(number)
  if day_format_lambda = @options[:day_format]
    day_format_lambda.call(number)
  elsif @options[:use_two_digit_numbers]
    "%02d" % number
  else
    number
  end
end