Glacier National Park, Montana is considered to be one of the most gorgeous national parks in the US. We went there in September 2020. It wasn’t snowing but the weather was still pretty erratic with random cold showers during the day. So, I would recommend going no later than August. Also, the east side, which has native American reservations were closed to prevent COVID-19 spread.
Dressing in at least three layers is highly recommended as the weather changes dramatically with heights and the time of the day.