Most industries are cyclical, and many value-plays come from buying companies at the bottom of this natural cycle.
A good time to buy is when you believe you are at the bottom of the cyle, and the time to sell is when you believe you are approaching the top of the cycle.
For example, commodities are notoriously cyclical. Extracting natural resources out of the ground requires huge captial expenditure and a considerable lag from implementation to production, and most projects only get going well towards the latter part of the bull market when prices have already gone up sufficiently to make these projects viable, but sows the seeds in creating the excess supply that causes prices to eventually peak. As prices then begin to fall lower and lower you can't just easily shut down an oil rig because of the huge sunk costs, so everyone clings on for longer and longer hoping outlast their competitors or the price to recover, and the continual overproduction eventually causes the price to fall through the floor. Many companies then go broke and production contracts, so creating the shortages in supply that cause prices to recover and drive the recovery, and the cycle repeats.