You can sow lathyrus when you sow regular peas, which is now. They don't transplant well, so sowing in place is a good idea, but I would cover them to keep them warm and insure good germination. You can also enhance that speed by soaking the overnight in cool water to get them fully imbibed.

This is what I do for peas:

1) Lay clear plastic over the area to be planted, with something underneath to create a mini greenhouse. This warms the soil

2) Soak peas overnight

3) Sow them and water them in

4) Keep covered until sprouted and growing on, though I'll uncover them if the temps get too warm

For me, peas grow faster than sweet peas, so I've taken to sowing lathyrus inside in peat pots starting in early March, trimming the seedlings so that they branch off. Then I transplant the peat pots. I get a better head start this way.

I sowed my sugar snaps last month and they are have some thick stalks poking out of the soil this weekend. In retrospect I would have soaked them ahead of time.