Create a "compact" human readable string output for enumerating multiple times/datetimes (modify according to your formatting preference/situation)

compact_format_for_multiple_times.rb

# For use with Time/DateTime objects
#
# Give it an array of Time objects and receive the following output as a String
# Examples:
#
# Sat/Sun 9:30am, 10am | Mon 9:30am, 10am, 11am
# Sat 9:30am, 10am | Sun 10am | Mon 9:30am, 10am, 11am
# Sat/Sun 9:30am
# Sat 10am | Sun 9:30am
# Sat/Sun/Mon 10am

# require 'active_support/core_ext/time/zones.rb' # If you want to test with Rails / ActiveSupport::TimeWithZone

def compact_format_for_multiple_times(times)

  times_grouped_by_day = times.sort.group_by{|time| time.strftime('%a')}
  # {
  #   "Sat" => [
  #       [0] Sat, 07 Jul 2018 09:30:00 UTC +00:00,
  #       [1] Sat, 07 Jul 2018 10:00:00 UTC +00:00
  #   ],
  #   "Sun" => [
  #       [0] Sun, 08 Jul 2018 09:30:00 UTC +00:00,
  #       [1] Sun, 08 Jul 2018 10:00:00 UTC +00:00
  #   ],
  #   "Mon" => [
  #       [0] Mon, 09 Jul 2018 09:30:00 UTC +00:00,
  #       [1] Mon, 09 Jul 2018 10:00:00 UTC +00:00,
  #       [2] Mon, 09 Jul 2018 11:00:00 UTC +00:00
  #   ]
  # }

  times_of_day_grouped_by_day = Hash[times_grouped_by_day.map{|k,v| [k, v.sort.map{|time| time.strftime('%k:%M%P').strip}]}]

  # {
  #   "Sat" => [
  #       [0] "9:30am",
  #       [1] "10:00am"
  #   ],
  #   "Sun" => [
  #       [0] "9:30am",
  #       [1] "10:00am"
  #   ],
  #   "Mon" => [
  #       [0] "9:30am",
  #       [1] "10:00am",
  #       [2] "11:00am"
  #   ]
  # }

  regrouped_days_with_same_times_of_day = Hash[times_of_day_grouped_by_day.group_by{|k,v| v}.map{|k,v| [k,v.map{|day_and_time_pair| day_and_time_pair.first}]}].invert

  # {
  #   [ "Sat", "Sun" ] => [
  #       [0] "9:30am",
  #       [1] "10:00am"
  #   ],
  #          [ "Mon" ] => [
  #       [0] "9:30am",
  #       [1] "10:00am",
  #       [2] "11:00am"
  #   ]
  # }

  # Sat/Sun 9:30am, 10am | Mon 9:30am, 10am, 11am
  formatted_output = regrouped_days_with_same_times_of_day.map{|days, times| [days.join('/'), times.join(', ')].join(' ')}.join(' | ').gsub(':00', '')

end

def test
  a_9_30 = Time.new(2018, 7, 7, 9, 30, 0) # Time.zone.parse('2018-07-07 09:30:00') # Sat 09:30am
  a_10   = Time.new(2018, 7, 7, 10, 0, 0) # Time.zone.parse('2018-07-07 10:00:00') # Sat 10:00am

  b_9_30 = Time.new(2018, 7, 8, 9, 30, 0) # Time.zone.parse('2018-07-08 09:30:00') # Sun 09:30am
  b_10   = Time.new(2018, 7, 8, 10, 0, 0) # Time.zone.parse('2018-07-08 10:00:00') # Sun 10:00am

  c_9_30 = Time.new(2018, 7, 9, 9, 30, 0) # Time.zone.parse('2018-07-09 09:30:00') # Mon 09:30am
  c_10   = Time.new(2018, 7, 9, 10, 0, 0) # Time.zone.parse('2018-07-09 10:00:00') # Mon 10:00am
  c_11   = Time.new(2018, 7, 9, 11, 0, 0) # Time.zone.parse('2018-07-09 11:00:00') # Mon 11:00am

  times_1 = [a_9_30, a_10, b_9_30, b_10, c_9_30, c_10, c_11]
  times_2 = [a_9_30, a_10, b_10, c_9_30, c_10, c_11]
  times_3 = [a_9_30, b_9_30]
  times_4 = [a_10, b_9_30]
  times_5 = [a_10, b_10, c_10]

  puts compact_format_for_multiple_times times_1
  puts compact_format_for_multiple_times times_2
  puts compact_format_for_multiple_times times_3
  puts compact_format_for_multiple_times times_4
  puts compact_format_for_multiple_times times_5
end

test()
# You should get:
#
# Sat/Sun 9:30am, 10am | Mon 9:30am, 10am, 11am
# Sat 9:30am, 10am | Sun 10am | Mon 9:30am, 10am, 11am
# Sat/Sun 9:30am
# Sat 10am | Sun 9:30am
# Sat/Sun/Mon 10am