It depends on the tour, as some routes run well above 2000 or even 2500 meters of elevation. Therefore, they are impassable in snowy conditions, limiting the visiting season to the summer and early autumn days between late June and early October.
However, some of our holidays keep to lower elevations, so hiking in early spring or late autumn is still possible. You can read more extensively about it on our subpage about the Dolomites.