Birch bark should be harvested in the spring when the sap is running in the trees. However, if done too late in the season the cambium (inner bark) will come off with the outer bark. If the cambium is damaged the tree may die. If done right, a tree will re-grow its bark over the course of 10-20 years. I have heard that the best bark comes from trees that have already been previously harvested, and traditionally people would return to harvest in the same grove over the course of several generations.