Any time is a good time to go to Death Valley. I’ve been there in every season of the year, for both volunteer work and vacation, and always enjoyed it. I think we can understand if you prefer not to go in summer, but that has its pluses too. But be aware that summer is a long season in Death Valley, because of the low elevations of much of the park and its unique location in the “rain shadow” of a series of mountain ranges. The Top Questions corner has a couple of threads about weather at different times of the year.
Spring starts in February-March and is a short season. It gets steadily warmer, peaks in July and August at 125º+, and cools off very slowly. The average daily high for every month from April through October is over 90ºF (over 100º from May through September). Only four months of the year have all-time highs lower than 100º, and it’s 98º for February and 97º for November. The park website has a section on weather where you can find maximum and minimum temperatures and precipitation for each month and for every day of the year.
Don’t let the warm weather deter you, because the humidity is typically very low and that helps your comfort level. If you come in one of the “shoulder” months, like April-May or October, the heat won’t be at its peak, although 100+ will still be common. OTOH, if you come in July or August, you’ll have an opportunity to share Death Valley with the locals and thousands of international visitors who flock to the park in summer because 1) they are required to take vacation during those months, or 2) they have been lured by the irresistible mystique of Death Valley and want to experience it at its most extreme.
Spring is wildflower season—if there is going to be one. It varies from year to year, depending on the amount and timing of rainfall over the winter. It can also be affected by how fast spring and summer come in; if it heats up too quickly and abruptly after the winter rains, there may not be adequate time for the plants to bloom. The park gives wildflower updates as needed in the spring; choose the Nature and Science section on the website.
Late summer (August to part of September) is usually thunderstorm season. When storms are brewing, it can be very uncomfortable, with the temperatures at 110º or more and humidity pushing 70-80%. It’s possible to have thunderstorms drop rain over the mountains but nothing at the Valley floor, because the humidity down low is still so dry that the moisture simply dries up in mid-air. When it rains in the mountains, you don’t want to be in canyons or dry washes, because flashfloods are possible and they can be deadly.
Rain is also possible in winter, and snow at higher elevations. The Panamint range, which forms the west wall of the Valley, goes up to over 11,000’ in elevation. Some of the most torrential rain I’ve ever seen in California was at Stovepipe Wells, the resort at sea level, in December. That same storm dropped enough snow on Daylight Pass Road, around Beatty NV, and along Hwy 95 north of Beatty, to require 4wd.
Dec.-Jan. are “less crowded” but this varies too. Winter holiday periods are popular for desert escapes, especially the periods around Christmas and New Year. In recent years, the balance has been shifting more to summer travel as overseas visitors come in greater numbers. But if you want to come in winter, I still suggest making reservations for any of the park resorts so you won’t be disappointed.
Because of the high elevations, stargazing is generally good. But like many Western parks, it sometimes gets air pollution that spoils the view. Death Valley can get spillover from urban smog in the Las Vegas and Riverside-San Bernardino metro areas. During summer wildfire season in California, fires in or near Sequoia or Yosemite national parks can mess up DV’s air. One of the popular spots for stargazing is Dante’s View, which is 5475’ in elevation. You will notice a dull glow to the southeast, which is not from the tiny nearby hamlet of Shoshone CA but from the lights of Las Vegas, 80 air miles away. Other good places are on the west side of the Panamint, along Emigrant-Wildrose Road, where there are high points and almost no human development to interfere with the view. In the north end of the park, the Racetrack (4wd high clearance may be needed) and Mesquite Springs campground are in higher elevations without much artificial lighting.
If you have the whole week for Death Valley, you can see many places that people don’t have time for in a quickie drive-by when they’re trying to rush from Las Vegas to Yosemite. You’ll have time for Scotty’s Castle, Rhyolite ghost town, some of the natural wonders in the north, and the Emigrant-Wildrose corridor. For backcountry exploring, you can rent a Jeep Wrangler from Farabee’s at Furnace Creek Inn, so you wouldn’t have to rent one in Las Vegas for the entire trip at premium rates. If you wanted to make a one-day or overnight side trip, Lone Pine and Mt. Whitney are two hours from Furnace Creek, and it’s a whole different world up there. Lone Pine has its own forum where you can find out what a delightful place it is. We’re all glad you have this much time to make it a more relaxing trip.