How can you possibly predict "good" weather 9 months in advance? I moved calving back from late Jan/early Feb to first week of March. And yet, there's always one that plops out a calf waaaay earlier than anticipated. And the unusual amounts of rain we've had the past few years? And the epic cold/ice/snow/hell that spanked half the country this year? Calving and weaning in mud is, IMO, worse than the cold. And, of course, calving later means weaning later - right when we're getting into questionable late fall/winter weather (because mine are weaned a minimum of 45 days). And that segues into selling them the absolute end of the year or having to hold them over, which can also affect our taxes, contingent on when we need to declare the income. Moving calving back further in the Spring in hopes of "good" weather also means turning out the bulls later. Can't speak for other bulls, but mine sure aren't at the top of their game when the heat index is 100, which then segues into a longer calving season (as opposed to my 6 week goal). I envy you that have spot-on, predictable weather.
You do what works best for your operation. And be as proactive as possible. And work closely with your vet. And always be prepared for Plan B.
Jumping down off my soapbox . . . . I've thought a lot about hitting my calves with Draxxin at weaning, but wondering if it's six of one, half dozen of the other. Calves have 3 rounds of respiratory, which should be more than sufficient. Is it $$$$ well spent? Would I come out even if I treated on a case-by-case basis? Because not all calves get sick. Or is this the year I'm back to zero respiratory issues and all good? I dunno.