i grew up in napa and i honestly believe it's at it's best during the fall when all the vineyards and the oak trees are a variety of colors.
as for the days the kids have off school-you can look at the napa valley unified school district website and it should show the calendar but in general the kids start around the first week of september, get maybe 3 days off in november for thanksgiving, 3 weeks off in december for christmas (may be back to 2 weeks now because when it changed from 2 weeks to 3 a few years back the parents were in an uproar), a week in march or april for easter, and then get out around the first week of june.
honestly-even if school's not in session the areas tourists tend to go are not impacted by the locals (wineries and such)-the traffic is pretty consistant year round (stupidly conjested in napa itself but the reality is that the areas most people associate with 'the napa valley' are not realy in the city of napa, they are in the adjacent towns of yountville and st. helena. sonoma is close by as well and has several wineries.
just by way of a suggestion/warning-if you are going to make any deposits on reservations or activities use a credit card with a company that will work with you if the buisness goes under and can't make good to you. the bad economic times have hit napa's tourist industry hard and every few days when i read the on-line newspaper for napa there is another restaurant or venue going under. copia was a very popular tourist destination and they closed up without notice a couple of months ago leaving people who had paid deposits or full payments for concerts and events with no recourse but to get listed with a massive amount of creditors on their bankruptcy proceedings (and the fallout to the other buisnesses in town that extended credit for services and supplies to copia has'nt come close to being fully realized
).
