Regardless of the season, walking on a glacier in Iceland is always a good idea. However, you do have to go with a professional guide because glaciers can be dangerous. Plus, you need proper gear.
My first time on a glacier was during the summer of 2016. Since then, I have been fortunate to be on a glacier many times and every experience has been uinque.
In winter, the ice is a stunning blue color and in summer you can explore more of these icy giants because of the better weather and longer daylight hours.