You see women wearing trousers for sports, particularly bicycling and riding, as early as the 1830s. That definitely didn't make them social acceptable, however. There's a story of a woman who wore trousers to the races in Auteuil, France in 1905. She had to be protected by police from the sort of mob it formed. Trousers became incredibly popular for beach wear in the 1920s and you can see plenty of women very purposely wearing the masculine style throughout the 1930s and 40s (Katherine Hepburn and Mae Wong were famous for it).

Over the 1950s and 60s it really became widely "acceptable". It certainly wasn't a sudden change. Through that time many schools still required girls to wear skirts as long as the floor when kneeling (my mother graduated in '66 and it was still the rule long after she left) and even some restaurants required women to wear skirts to be served. You do see shorts becoming prevalent as well during this time, particularly so for younger girls. However, remember that these garments are still considered very, very casual. Trousers and shorts might be worn for activewear, the beach, camping, or other activities, but not to work in an office, attend church, or even a party at the neighbors. That final push to make trousers everyday mirrors the women's rights movement of the 60s/70s.