I read the excellent article on how to solarize raised bed gardens, but it didn't say when is the best time to do that. It says for how long, 4-6 weeks, which makes me think you do it 4-6 weeks before planting, is that right?
Solarizing Raised Bed Gardens
Certified GKH Gardening Expert
Yes, solarization can be done before planting in the spring. The most effective time to solarize is during the hottest part of the summer, since this will kill weed seeds and soil pathogens even a few inches down in the soil. Of course, that is also the time when most people are using their garden to grow plants, so summer solarization is only practical if you have a large garden and are able to sacrifice a portion of your space every year.
However it can also be effective to solarize for 4-6 weeks in the spring before planting and in the fall after harvest. If you choose a black tarp or other cover, the lack of light will also help kill weeds that sprout. Here is more information: