I actually bought a house in December, but on average you want to try and maximize your opportunites which at times can be influenced by the time of year. An example might be the sale of a house prior to a school year starting. Spring also seems to be a decent time after everyone has been cooped up during the winter and they begin driving around. However, a buyer's motivation will always trump timing, in my opinion.
As to whether it matters if a house is renter occupied, vacancy has its advantages from a maintenance and improvement standpoint, but occupied is what most investors interested in buying the house are looking for. It's always beneficial to be able to go to a lender and show an income stream for a property they are considering financing. There are some things that a buyer need to consider with an occupied rental and they should always consult their attorney to be certain the tenants rights are not violated in the exchange of ownership. Good luck with your decision.