Ideally you want your cows to have their calves at a time when they are going to have plenty of grass to eat to make lots of milk to feed their calves. In temperate climates, springtime is ideal as spring rains will bring grass and the calves will grow fat over summer. For us, spring can often be the driest time of year, and summer brings drought, heatwaves, or flooding rain and paralysis ticks. Sometimes we have lots of green grass in summer, but its not reliable.
In the sub-tropics, autumn is cooler, but also tends to be drier. We find that grass is starting to go to seed, dry out and does not have as much nutrition as green grass. By winter we've usually had some frost and our tropical grasses are all dead and dried out. There's really no great time for us to have calves, but autumn is a bit better than other seasons and we always make sure we have hay ready to feed if the grass is too poor. Of course, all of this is different if you have irrigated forage to feed your cows!
With our dairy cows we don't worry so much about the season, as we can help our cows look after their calves. However, we really have to think ahead 9 months and make sure that we will be available to milk the cow twice daily for at least 6 weeks after she calves. Again, its difficult to look ahead 9 months and predict whether it will be a good time to have a calf!
One more thing, cows tend to hide their calves in the grass and pretend they don't have a calf. This can make it difficult to find them!