At evenings/at evening is not at all wrong or as rare as many foreign learners think. It is often used interchangeably with in the evening(s), but is rather preferred when the time referred to is not specified, as in your example. The expression at evenings and weekends, in particular, is more or less a standard one. The use of the preposition at actually refers to time. In the evening(s), on the other hand, seems to imply more a connection with some place or physical surroundings as they look in the diminished evening light. For example, we are usually taught that we must say at night, but in the night is far from being wrong or rare. Just think of the famous Frank Sinatra song, Strangers in the night! So, in the evening/night mostly describes a visual conception (of how people, buildings, objects etc. look and/or act in the dark), whereas at evening(s)/night describes a time part of the day when something happens instead of (or in addition to) some other part of the day. The distinction, however, is a very subtle one, so people will often use one or the other construction without really thinking.