According to many expert opinions I've heard or watched,  the "BEST" time to calve is 30 days ahead of your peak grass growth, whenever that is in your climate/location. 

The idea is 30 days after calving is when the cow is at peak lactation to meet the growing calf's needs and the calf is not yet mature enough to do much grazing on his own. Almost all of the rapidly growing calf's nutrition comes from the cow's milk around 30 days after calving.

around here in SW WI peak grass is usually about the first week in June, so, according to this approach, calving should be the first week in May, Given natural/bull fertilization, not all calves will be born exactly the same day or week so I start calving the first of May or maybe a week earlier. 

This method (30 days ahead of peak grass) tends to avoid the wild weather ups and downs of March and early April. Generally with May 1 (that's HERE, your 30 days ahead of peak grass is likely different) calving and cows, and especially bulls, with good CED EPDs all you have to do at calving time is to count them. Let  the cow and nature do their job. And you are free to concentrate on crops.

It was 20 degrees F this morning here with high humidity and frost on everything. I sure would not want to be calving now unless I was set up with sufficient indoor facilities as I've seen on a few farms in SD.

Using the AHA Hereford gestation chart, this means a bull-in date of July 18 +/- a few days, but again this is for MY location, not necessarily yours.

Jim