output.rb

>> Time.parse("3/30/2010 3:30PM").to_s(:event)
=> "Tuesday, March 30th @ 03:30 PM"
>> Time.parse("3/25/2010 3:30PM").to_s(:event)
=> "Thursday @ 03:30 PM"

some_initializer.rb

EVENT_START_DATE = Time.parse("3/24/2010")
EVENT_END_DATE = Time.parse("3/29/2010")

ActiveSupport::CoreExtensions::Time::Conversions::DATE_FORMATS.merge!(
  :event => lambda do |d|
    if (d > EVENT_START_DATE) && (d < EVENT_END_DATE)
      d.strftime("%A @ %I:%M %p")
    else
      d.strftime("%A, %B #{d.day.ordinalize} @ %I:%M %p")
    end
  end
)