How to Plan Your Trip
As mentioned previously, wine country is divided into two valleys: Napa and Sonoma. There is a mountain range in the middle that makes going back and forth between the two very difficult.
If you only have three days to spend in wine country, choose a valley. If you have five or more days to play with, you can make a loop by picking one valley, traveling north, and then crossing over at the top, and heading back south.
Seasons
Wine Country is fabulous no matter the season but the most popular times to go are spring through fall.
Spring = bud break! This usually happens in March or April. The mustard also blooms in-between the vines which makes for particularly beautiful scenery.
Summer = hot. Probably Rachel's least favorite time to go. Temperatures will get up into the 100°s and traffic is at its worst.
Fall = harvest. Possibly wine country at its prettiest. The bunches of grapes are huge and the leaves change color like the foliage back east. Harvest begins around the first week of September, depending on weather.
Winter = rain. Being the off-season, you may be able to get some good deals on rooms and the tasting rooms will be less crowded meaning your pourer is more likely to spend more time with you.
Notes: check your itinerary to make sure you're not going up the same weekends as popular festivals such as passport or Bottle Rock.
Day Itinerary
Traffic in Napa in particular can be stifling on Rt. 29 and 128. If you're staying in the City of Napa, it is best to start as far north as you're going that day and then work your way south. Similar advice applies to CA-101 in Sonoma.
Plan on visiting three wineries a day. You can maybe hit four but don't rush yourself! Most wineries open around 10 AM. So you may be able to get in one tasting before lunch. Most wineries close around 4 PM so odds are you'll get in one or two after lunch as well.
At this point, you'll probably be exhausted! So head back to the hotel for a nap and a shower before dinner.
Note: times in Sonoma County are generally pushed back by 30 minutes to 1 hour.