May will be wet and spectacular. The falls will be roaring this year from the snow melt. To give you an idea of just what kind of snowfall we get here in the Sierra Mountains, Squaw Valley, a ski resort near me is reporting over 330 inches of base. Yosemite will probably be half that but still tons of snow. I would do a Google on Yosemite horseback trips to see what comes up. I have hiked much of the park even from Lake Tahoe to the north valley (The Tahoe/Yosemite Trail). Do get down to the south end of the park where the Giant Sequoia grove is, the trees are awesome. May is a great month, far less folks there, its still chilly and you will see falls that will take your breath away. My very first site of Yosemite was El Capitan, wait till you see that. The hike to Half Dome is well worth it. Have good waterproof boots and a rain slicker. The Misty trail, if its open will get you wet from the falls mist. Yosemite, it is still the most awesome place I have been and that includes Denalli park in Alaska where I hiked to the base of McKinley, the Grand Canyon, Zion, Bryce, Flaming Gorge, Jackson Hole, Glacier park, every park on the Oregon coast, Acadia park in Maine, Big Sur, Point Reyes, I mean I loved all those beautiful places, but Yoemite is magical, I still simply cannot believe it when I get there it is that awesome.