hmmm.... as long as there is something that can be identified as movement of air masses you can sail in it... ... on lakes, even on still or calm days you will have some kind of movement from thermo bubbles rising over land sucking cool air from around the lake. That type of wind might only last for a few minutes and wind direction changes all the time.
In general you can sail with almost nothing... stuff gets boring and your a$$ falls asleep a tack might take a minute to complete but it works. In my region the forecast calls for about 5 miles/h this weekend. Not good enough to get me to the lake on Saturday.

Patrick