April is the peak month for the Tulip Festival and worth planning a trip around if flowers are your thing — but expect crowds and accommodation prices to spike accordingly.
Summer (June to September) is the most reliable season for visiting the North Cascades; Highway 20 is open, the days are long, and the weather is usually dry and clear. Temperatures in the valley hover in the low 20s Celsius at peak summer. The mountains can still see cold and rain even in July and August, so come prepared.
Winter brings cloud and rain to the valley (the Cascades wring most of the moisture from Pacific storms before it reaches eastern Washington). Highway 20 closes over the mountains. Not the ideal time to visit unless you're heading to the ski areas on the other side of the pass, which are accessed from the east.