You can order at the beginning of March (best not to get them too much earlier than that) and have them in a box (cardboard, rubbermaid, anything that's big enough) indoors or in a shed for the first 5 weeks. Make sure to do the heat lamp--they need at least 80˚F the first week, preferably closer to 90˚F (the weak ones won't make it under 95˚F, and cornish cross are usually all pretty weak... ranged and slower growing meat birds can be hardier). By week 5 they can go down to 50˚F if you've been slowly dropping the temp every day, but they can't get wet or windy. By week 8 they should be fully feathered and good down to 32˚F if they've been off the heat lamp for a good week or so (not that this would happen in SoCal in May), still try not to get them soaking wet at any point in their lives and provide cover from the wind.