As a real estate broker, this is the number #1 question I get time and time again. The answer is... it depends.
Real estate is very cyclical and generally speaking, home prices tend to creep up as we approach the summer selling season. Many sellers wait to get their home listed around summer time to maximize their sale price, and housing inventory rises as a result. Therefore listing your home between the months of May and August can help maximize your sale price; however, there will be more homes on the market that your home will have to compete with. If your home is updated and has what buyer are looking for then you can maximize your sale price. However, if your home needs updating and repairs, then your listing may get stale as buyers pass up your home for homes in better condition.
So if you are thinking of selling and can wait to list your home during the summer months then go for it! Just remember you will be competing with an increased number of other sellers that have the same timing strategy. Also remember that if your selling your house and buying a new house at the same time, the new home will have also increased in price so it may be a wash.
Prices tend to stabilize or dip during the holidays, however; so does housing inventory. This means that buyers may get a better deal on a home but it may be slim pickings as far as what there is to choose from.
Timing the market is a tricky thing and can't always be predicted. Many factors influence housing prices and inventory such as the general state of the economy, interest rates, investor activity, etc. However, if you do try to time the market then keep these general trends in mind!