The State Opening of Parliament takes place after a general election and at the beginning of each new session of Parliament. Since 2012 the Government has given an undertaking that State Opening will happen in the Spring, although in the past it was generally set for November.
Pre-2012, after a general election, the following parliamentary session was a long one to allow enough time for legislation to go through. As the House had a break during the summer, a November State Opening of Parliament in the same year as a general election in the spring would leave only four or five months for legislation to go through. Therefore, the next State Opening after an election was not until some time around November the following year.
Further information on this subject can be found from the following links.
About Parliament: State Opening of Parliament
Related glossary term: Session