I sponged my ewes last year and it worked very well, as they all got tupped over a two-day period, and then lambing was really tight too. (It would have been an all-round success if tuppy would have been firing on all cylinders, but alas he didn't and I had only 50% of the white ewes (all three of them!) lamb, the other three didn't, despite being tupped twice.)
Depending on your ewe numbers and the age of tup you are buying, insert the sponges (all in one go or in two batches), and let the ewes run in a field next to the boy during that time. Remove sponges on day 12 (I think) and then let tup into that field two days after. If you manage to put some raddle powder on his chest, you can see if he does his whole flock within 24 to 48 hours, which is what he should do. Then leave him in and change colour 2 weeks after he first went in (ewes cycle is 17 days) and then you see if he is repeating on any of the girls.
Lambing should all be over within a week, some 140something days later.
So sponges go in 145 days plus 2 days plus 12 days before you want to lamb. My average has usually been around 146 days gestation.
If you lamb outside I would think the later the better, I now try and synchronise with school holidays in April, or if too early aim for last week in April. My ewes are then penned up in pens (Tim Tyne's design) straight after lambing, nowadays outside, but used to bring them into the shed (now got billy in residence), for about two days.
Shetlands prefer to lamb outside, mine were given the choice and usually went out. The commercials would stay inside! Most of them will start at dawn (so 5am start) or just when you get the dinner on the table...
How big is your field? I normally cordon off a largish corner of my lambing field, where the pens are too. Then at night I bring all the girls into that pen (6 x 6ft hurdles square), it means that I can check quickly at dawn and dusk (if necessary with a torch) from some distance and don't need to go near unless necessary. I also bring the newly lambed ewes and their lambs back into that pen for a couple of weeks or so, as we had a lamb killed by fox/badger a couple of years ago. Smaller pen means ewe has got more chance of NOT loosing her lamb.