You want to avoid postal holidays any time of the year. You will notice more complaints about shipping delays on this forum around holidays.
The problems generally do not come when things are normal for the time of the year. They are packaged in a way that they can handle normal conditions, even in colder or hotter weather. Their routing and handling is set up to avoid extreme conditions. Extreme conditions: winter weather, hurricanes, tornadoes or such can cause shipping delays. Those are never good but are relatively rare. If you can ship them when the chances of an extreme event is less likely your odds are better.
Another potential problem is when they are not handled the way they are supposed to be. There are procedures for shipping chicks. But if a box of chicks is left exposed to a winter storm when they are supposed to be somewhere protected or maybe left in a locked truck in the hot sun you can have serious problems. If they are not having extreme weather and something goes wrong there is less chance of it being anything bad.
Yours will be flown to you. The odds of a flight being delayed or cancelled or extreme weather will be less in April than in March. You will probably be OK with a March shipment, most are. Your odds of a successful shipment in April are not 100% but they will be better than a March shipment. If you can get what you want and are OK with the later date I'd suggest delaying a month. But do not delay into the hot weather months.
