Actually there is no one good time to visit Alsace. It has a pretty mild winter and a somewhat hot summer (with a few really hot days here and there).
You can really benefit from the smaller numbers of visitors in the late winter and early spring.
Spring brings the flowers, as well as White Asparagus season.
In early summer you have cherries, strawberries and raspberries in season and of course the geraniums everywhere, but more people.
Along with that comes the fresh fruit sorbets and ice creams too!
In the summer you have more visitors and nice sunny weather.
Than in the Fall, you have the harvest which can be an obstacle if you are coming to do wine tastings, but it is quite beautiful.
And you also have the cabbage harvest and the Choucroute is ready! That's our Alsatian sauerkraut which is unbelievably good (even for people who don't like cabbage).
And to finish the year, the Christmas season starts the last week of November and goes to the end of December.
It's a very special time of year in Alsace and well worth your time.
So really there are so many times you can come to Alsace : )