New Zealand will go to the polls on September 20.
He said G20 summit hosted by Australia on 15-16 November was a factor in deciding the date.
Australia has invited New Zealand to take part in that summit, and Mr Key said there was a risk holding the election too close to the G20 would meant post-election coalition talks were still underway and a prime minister had not been officially anointed.
An election campaign could also mean other country's leaders could not visit in the lead up to that summit.
He said Parliament would rise on July 31.
Labour leader David Cunliffe has expressed concern about the hosting of the visit of Prince William and the Duchess of Cambridge in an election year, saying there was a convention for a gap of at least six to eight months between a Royal visit and an election.
In 2011, Mr Key announced the election date in early February - nine months before the November election. It is up to the Prime Minister to decide on the date.
The regulated period for election spending and advertising now kicks in either three months before the election, or after the Prime Minister's announcement if that is within three months of the election.
Traditionally prime ministers have left it late to call the election to take advantage of knowing when the regulated period will start. However, Mr Key said he does not necessarily agree that delaying it until the last minute gives a big advantage and it is in the interests of the voters and economy to have some certainty early.
Mr Key is also expected to give clearer indications of whether National will assist potential coalition partners in electorate seats closer to the election.
He has already indicated it will do so for Peter Dunne in Ohariu and Act's Seymour in Epsom, but has not yet said whether it will support Conservative Party leader Colin Craig in the same way. National ministers Paula Bennett and Murray McCully have both said they are interested in the two seats Mr Craig is most likely to be successful in - the new Upper Harbour electorate and East Coast Bays.