The Wonderland Trail is most commonly hiked during the months of July, August, and September:

• July marks the transition from winter conditions to great hiking, but the melt date varies greatly from year to year. Risks include deep snow at higher elevations, consequently difficult route finding, and raging rivers where missing bridges may not yet have been replaced. If you make reservations for early July, be prepared either with the skills for safe snow travel, or to cancel at the last minute if the mountain does not melt according to your schedule that year.
• August is the most popular month, and for good reason. Best weather. Best wildflowers. Perhaps still some snow on the high passes, but most of the trail will have melted. Most people. Most mosquitoes.
• September offers fall foliage colors, fewer bugs, and highest probability of the whole trail being in good condition, but also cooler nights and some risk of early fall snow. There may be (slightly) less competition for permits, as many cannot backpack during the school term.

Some people do complete the trail in June or October (or even beyond!) but these months greatly increase the odds of encountering winter conditions or dangerous river crossings.