Scott... "Spins" are day duties in lieu of nights. As more staff are needed during the day each watch splits for the last two days of each shift - some guys work the nights and the others work days (spins). "D2" simply indicates the type of "spin" duty. A D1 might run from 0700-1300, a D2 might run from 1000-1800, etc.
The general question of days and nights is very interesting.. I have always despised nights throughout my life but have managed to avoid as many as possible through swaps with colleagues who prefer them. During my 23 years at Heathrow I once went for over 8 years without working a night duty. But I know of several colleagues who much prefer night duties and have only worked a handful of "spins" in 30 years. Once we moved to West Drayton things changed because of the small number of controllers working the Heathrow sector so the nights came around much more frequently. That was the prime reason for me changing to a part-time contract (4 days on 6 off). If somebody had said I could work all days and no nights for a small reduction in pay I'd have done it!