Keep an eye on your weather forecasts. I've watched them hard the last five years now, I am in one of the warmer areas of the country
- there always seems to be a cold snap in April, there has been in May as well but not frosts, just very cold, too cold for cannabis really (anything below 40 is too low really over sustained periods - although the coldest nights usually go with the warmer days in April/May). Thing is those spells don't usually last long, so if they've had a month or so of indoor veg and some acclimatisation to outdoor conditions (like put em outside 2 hours a day, then 3 etc etc) then I think they will be ok from April. My mate swears by vegging indoors under sun alone (in a window), I find the plants just whither away into nothingness, and desk lamps don't do the job - try and convince your dad to give you 4 weeks grace with a CFL, tell him about it being veg and no smell (or actually cannabis, eg thc), and that in a month they will be gone. See how you get on. So April is probably safe, depending on the strain and luck with climate you could probably get away with mid March, but you're risking it if we suddenly get two weeks of too low temps.