The best time to sell is really when you are ready or when it becomes necessary to sell. That is, when you have outgrown the space in your current home, or you prefer to trade down to something smaller. Perhaps your personal status has changed, which necessitates a move, or you need to relocate for a job.  Other factors to consider are market conditions which also play a role, as do seasonal conditions.  For example, your chances of getting top dollar for your home are more likely in a seller's market, when demand outweighs supply, than in a buyer's market.
Local and national economic factors also may dictate when to sell. If the area is suffering from business downsizing or reducing staff, it may not be a good time to put your home up for sale. People will be cautious about buying when the future seems so unpredictable.  However, there are seasonal periods where selling a home is considered a Good Time to Sell.  In the spring is when most agents agree is a good time.  This is when the largest number of potential buyers hit the market. Your home is likely to sell quicker and at a higher price. Statistically looking at Fortbend County for example the 77469 zip code January through July show higher closings.  Similar results may been seen throughout the Greater Houston metropolitan area(s).