BHI says - "Spring" can be wet and cold. Mid-May to Mid-June is black fly season. My favorite time is late June or Sept to mid-Oct. In the fall their are less families and more retirees especially cruise ship passengers so you'll find less people on the trails. Sept weather is great for hiking, warm during the day but cool at night. Pack some warm stuff. The shops in Bar Harbor do brisk business in sweatshirts when people from away come without proper clothing for the cool ocean breezes at night.
When to Go to Acadia National Park
Memorial Day and Labor Day mark the official beginning and end of high season on Mount Desert Island. The reality, however, is that there is no reason you have to visit during this narrow window. Temperatures often begin to rise in April or May. The crowds are smaller during these months, but you may have to contend with minor irritants such as ice and snow on the trails through Acadia National Park.
By September, the heat and humidity of the summer begin to taper off, making it one of the most enjoyable months to visit. Autumn foliage peaks between the end of September and the middle of October, enhancing the already spectacular views. Although many seasonal businesses close their doors after Columbus Day, the island does not shut down entirely. You can still find some good restaurants and a small number of lodging options throughout the winter months.
Regardless of when you decide to visit, you will enjoy your visit to the island more if you book your accommodations in advance. Although it's possible to find last-minute accommodations during the summer months, it may take several phone calls before you locate a room. If you have a particular type of lodging in mind, it's best to book as far in advance as possible. Accommodations tend to fill up especially quickly on holiday weekends. From November to April, your challenge shifts from finding an open room to finding an open hotel. You'll definitely want to call ahead.