The best time to climb Mt Rainier is essentially the month of July, as this month tends to see the most stable weather in the upper elevations. That being said, the general climbing season for this formidable peak to the near southeast of Seattle begins in April and extends through the month of September. The rest of the year is pretty much winter on the Mt Rainier slopes, and heavy winter storms are very common. This basically means that anyone who is thinking of attempting a winter climb to the summit should be highly skilled in the art of mountaineering, especially as it relates to wintry conditions.
Mt Rainier rises 14,411 feet into the sky and is the most prominent peak in the contiguous 48 states. This, coupled with the fact that it is largely covered in glaciers, makes it a difficult climb. In fact, of the more than 10,000 people who attempt to summit every year, only about half make it to the top. Weather and fatigue are the most common obstacles regardless of the season, and it almost goes without saying that anyone who is wishing to reach the summit should be in good physical shape at the very least. As for the reward for those who do reach the summit, the views of Washington and Oregon unfolding below are utterly amazing, especially on clear days.
From the common starting point at Paradise (an area on the mountain with a lodge and tourist activities), climbers face a 9,000-foot ascent if they wish to reach the top of Mt Rainier, so that should give you an idea of the task at hand. The most common place to take a break before attempting to complete a summit attempt is Camp Muir, which sits at an elevation of 10,188 feet. The "town" of Paradise, on the other hand, sits at an elevation of 5,400 feet. Other camping options exist and are taken advantage of by most climbers, as it typically takes two or three days to complete a summit climb.
The general rule when it comes to camping on Mt Rainier is to stick to bare ground areas that were previously used as campsites or to camp on permanent snow or ice. Reserving certain campsites in advance is possible and can be an especially good idea during the busier May through September season. Mid-March is when the Mt Rainier National Park begins accepting camping reservation requests.
While summer is considered to be the best time to climb Mt Rainier, summer climbers should expect to encounter plenty of challenges. In fact, during the summer, the upper elevations can be hit by snowstorms that you might expect to encounter in the dead of winter. An increased amount of crevasses tend to open up during the summer as well, and rockslides are usually more common. Due to the difficulty overall, booking a guided climb is the recommended way to go for most people. There are various operations in the area that offer guided summer climbs in particular, and more often than not, these adventure organizers also offer mountaineering classes that can come in very handy for the less-skilled climber. Seattle and Ashford are good places to start when looking to book a such a tour or class.
As a side note, anyone who is planning on climbing Mt Rainier should know that a Mt Rainier Climbing Pass is required for those who ascend more than 10,000 feet. You will also need one of these passes if climbing on glaciers is part of the plan. As for where to obtain a Mt Rainier Climbing Pass, the locations, which vary according to season, include the Longmire Museum, the Carbon River Ranger Station, and area visitor centers such as the Jackson Visitor Center.