Yosemite's waterfalls (mostly in Yosemite Valley) usually peak in mid- to late-May, so this is the best month to see them. While it may seem like summer, snow is usually present in much of the park, and the Tioga and Glacier Point Roads may be closed.
It's possible to enter the park via Highways 41, 140, and 120 from the west (these roads are open all year). Hetch Hetchy Road is also open all year. The Mariposa Grove Road is usually open in May.
The Tioga Road (Highway 120 through the park) and Glacier Point Road are closed, typically until late May or early June. While it's impossible to predict when these roads will open because April and May weather significantly affect plowing, you view a list of opening dates since 1980 to get a feel for when they can open.
Arrive at Yosemite before mid-morning, especially on weekends, to avoid delays at entrance stations and popular areas like Yosemite Valley and Glacier Point.
Tire chains are not normally required in May, although the occasional May snowstorm could result in chain requirements. You can check on current requirements by calling 209/372-0200 (then dial 1 then 1).
Yosemite Valley is the most popular area all year and especially in winter and spring. In the Valley, you can see Yosemite and Bridalveil Falls, among others, and various rock formations, such as Half Dome and El Capitan.
Hetch Hetchy, a valley similar to Yosemite Valley that was dammed, is generally snow-free and Wapama and Tueeulala Falls are full.
Mariposa Grove Road is normally open in May, allowing for access to vehicles displaying a disability placard, with free shuttle service between the Mariposa Grove Welcome Plaza (near South Entrance) and the Mariposa Grove.
Weather in May is usually warm during the day but cold overnight, with average highs in Yosemite Valley in the 70s and lows in the 40s. While rain and snow are possible, they're not very common.
Hazy or smoky conditions may occur even during spring. In some areas, the park may have prescribed fires to mimic natural fires to promote forest health and reduce the risk of larger fires; conditions in spring are often good for such fires. Less common, but possible, are fires in the region or in the park. Lightning-caused fire occurs naturally in the Sierra Nevada. Additionally, accidental and arson fires can also occur, but firefighters typically attempt to suppress these fires immediately.
Hiking and backpacking options are still limited, even in May. Expect snow above 7,000 feet (this varies from year to year). Day hikes from Yosemite Valley are usually snow-free, but trails along the northern and southern rims (e.g., El Capitan-Eagle Peak-North Dome and Pohono Trail) tend to be snowy and/or flooded. The Four Mile Trail to Glacier Point is closed, usually until late May, but may be open to Union Point, about halfway up.
Outside of Yosemite Valley, trails in Wawona, Mariposa Grove, and Hetch Hetchy are usually snow-free, but may be wet.
Few multi-night backpacking options are available in May, depending on conditions. The trails in best condition will be those leaving from Yosemite Valley, Wawona, and Hetch Hetchy, but it's unlikely you'll be able to find a hike more than two nights long that doesn't involve significant snow. Wilderness permits are required and may be difficult to get.
Camping without a reservation is extremely difficult in May because the only first-come, first-served campground open is Camp 4, which is a walk-in campground with shared campsites. Camp 4 fills up early each morning.
The Upper, Lower, and North Pines (Yosemite Valley), Wawona, and Hodgdon Meadow Campgrounds are on the reservation system with no sites available the same day.
In Yosemite Valley, Housekeeping Camp, Half Dome Village (formerly Curry Village), Yosemite Valley Lodge (formerly Yosemite Lodge), and The Majestic Yosemite Hotel (formerly The Ahwahnee) are open.The Big Trees Lodge (formerly Wawona Hotel) is also open.
Biking: Bicycle paths in Yosemite Valley are snow-free in May and an excellent way to get around eastern Yosemite Valley. Bikes are available for rent at Yosemite Valley Lodge (formerly Yosemite Lodge) and Half Dome Village (formerly Curry Village).
Rafting: The Merced River is usually closed to rafting and kayaking in May, except following relatively dry winters.