Yes just had same email and it is just between specific dates in March (3rd - 27th inc.)
perhaps more importantly, specific crossing times on some days/dates.
This offer is available on the following departures between 3 & 27 March:
All departures between 23.00 & 06.00 inclusive in both directions
All departures from Dover from 16.00 on Saturdays until 23.59 on Sundays
All departures from Dunkirk or Calais on Fridays and Saturdays
PS Booking page here.