They can be born at any time of year.
However, many dairy farmers make sure their cows calve mainly in the autumn.
On the other hand most single-suckler beef cows (that is, cows who rear their own calves) calve in the spring, say from April to June.
Herds are therefore described as "autumn calving" or "spring calving"
Commercial farmers usually try to get all their cows calving at much the same time.
Our own (not very commercial) single-suckler cows calve mainly in early summer, but we have a few at other times of year. This year we had one in January, about ten in June and July, and there'll be another three or so in November and December. We prefer them in the early spring.
As successive calves are usually born a year apart, once cows start they will tend to calve at the same time each year -- you can only change them by keeping them away from the bull for a while (not easy...), and so losing part of a calf's-worth.