Humans are remarkable in many ways. Most of us, for example, have sex in private. Compare that to most other mammals, who will copulate in clear view of their fellows.
Female mammals usually have a well-defined period of time when they are sexually active. In most primates, at ovulation, the skin around the vagina swells and changes color, and the animal's odor and behavior change. Sex occurs during this estrus period and may be prolific (female baboons copulate about 100 times with many different males).
Humans are different. There are no morphological or behavioral changes associated with ovulation (or such changes that do occur are subtle compared to those of primates like baboons, who waddle around with heavily swollen pink hindquarters raised provocatively in the air). In humans, the 28-day cycle of ovulation is concealed from onlookers. There is no specific period when sexual activity occurs -- humans have sex continuously.