Hiking in the Alps is typically possible from mid-June to late September, depending on the snow conditions at high elevations. Many hiking routes cross mountain passes that are located above 2000 meters, so it is important to check the current conditions if you plan to visit in early summer.
Keep in mind that the huts along these routes close for the season outside of this time frame, so it is not possible to plan multi-day tours outside of these months.
Read more about the hiking season on the Alta Via 1.