I'll direct you to Soranus, Gynecology 1.36:

Just as every season is not suitable for sowing seed on the ground for the purpose of bringing forth fruit, so too among humans not every time is suitable for the conception of seed ejaculated during intercourse. Therefore, in order that the desired outcome may be attained through the proper timing of intercourse, it is useful to discuss here the problem of proper timing. The best time for intercourse resulting in conception is when menstruation is ending and abating.

Soranus wrote in the first and second century CE. What is striking about his work is that he noticed that there was a rhythm (albeit for purposes of conceiving children) however he missed what modern science considers the fertile period by about 7-9 days. Even so, Soranus suggests that people who are not looking to conceive children reserve sex for less fertile periods of the menstrual cycle.

Its likely that these ideas were developed and tested before Soranus. If I remember correctly Lucretius (1st C. BCE), in his De Rerum Natura IV, discusses one cause of sterility as not having intercourse at the right time. These ideas likely stem from the gynecological work of Hippocrates in the 4th C. BCE, though my knowledge of his sexual corpus is lacking. You might want to start there if you are interested in researching the subject further!