There are several questions to ask before you can answer this question. Do you want to go during the harvest season when the vineyards are busy and buzzing with activity? Would you prefer to go during the off-season when prices are better and the crowds are less?
The best time to visit Napa is September through November or March through May. Napa's peak tourist season corresponds with the region's harvest season (August through October). During this busy time, expect high prices for everything, including food, accommodations and wine tours.
For me I like to go when the weather is nice, but the crowds are a little less than the busiest time of the year, so I've decided to do a guided tour of Napa Valley in April 2019.
Currently we're working with the logistics of this tour but that should be done very soon. If you're interested in knowing the details let me know and I'll send them as soon as they're available.
Send us a note and you'll be included in all of the updates for this tour.