Create a calendar (array of weeks, each an array of days).

calendar.rb

class Calendar

  attr_reader :date

  def initialize(date)
    @date = date
  end

  HEADER = %w(Sunday Monday Tuesday Wednesday Thursday Friday Saturday).freeze
  START_DAY = :sunday

  def weeks
    first = date.beginning_of_month.beginning_of_week(START_DAY)
    last = date.end_of_month.end_of_week(START_DAY)
    (first..last).to_a.map do |date|
      {
        classes: day_classes(date),
        date_utc: date.to_time.to_i,
        date_formatted: date.strftime("%b %d, %Y"),
        day_of_week: HEADER[Date.today.wday]
      }
    end.in_groups_of(7)
  end

  private

  def day_classes(day)
    classes = []
    classes << 'today' if day == Date.today
    classes << 'not-month' if day.month != date.month
    classes.empty? ? nil : classes.join(' ')
  end
end