Though he painted it over time, and spent a lot of time studying and planning, he was depicting a single, though fictional, moment in the park. It is a SUnday afternoon because that is what he is depicting socially -- a range of working, professional and upper class people are in the park on the Sunday afternoon. People who have been to church are there. For many people Sunday was the only day of leisure. His ambition was to show a whole society in that moment, and at any other time no such cross-section of society would have been present.
