This is a crowded time to visit the holy city of Haridwar because of the school there are holidays all over India but this is a great time to take a dip in the holiest of all rivers in the world, and the one of the few rivers that is prayed to so reverently as the waters are not so cold. The water in the Ganga here is directly coming from the melted snow of the mountains above.
This is a very nice time to go because a lot of the summer rush is missing. Also the weather is much more bearable now and it is just beginning to get a bit cooler. Most of the activities that one can only enjoy in the summer can be done at this time including the Char Dam Yatra.
This is a very pleasant time to go to Haridwar but the weather can be quite cold and hence dipping in the Ganges which is the main attraction can be difficult as the water is so cold that it is difficult to stay inside for too long.
Spring or the months around February to April are much warmer than the winter months. There are some major festivals during this time like Makar Sakranti and Maha Shiv Ratri which are celebrated in a grand way in Haridwar.