Is there such a thing as the right time to start a start-up? Generally speaking, there is a right and wrong time. You want to be able to execute on an idea when the market is willing to accept. If you go too early then everybody will have the WTF face whereas if you wait too long the ship will sail and you'll be kicking yourself.
So let's put aside the market conditions and see if there's a specific time of the year to start a start-up. It's always been an accepted statement that "yesterday" was the day to start your company, but is there any truth behind that?
I looked at the dates (with linked sources) of a few companies to see if there was a trend for success:
eBay: September, 1995
Google: September, 1998
LinkedIn: March, 2003
Facebook: February, 2004
Digg: October, 2004
Meebo: February, 2005
YouTube: September, 2005
Mint.com: Launch September, 2007
Twitter: March, 2006
Groupon: November, 2008
FourSquare: March, 2009
These are only 11 data points, but they're significant companies that you shouldn't ignore. Three out of the four companies (eBay, Google, YouTube) that have already exited were founded in the magic month of September. Mint.com's exact founding date is unclear, but they launched in September 2007 at TechCrunch40.
The two sure bets on an IPO (LinkedIn and Facebook) decided to found their company in the late winter and early spring time. Everybody else is scattered through the spring and fall.
December and April-August seem to be the quiet months. It's likely because of the winter and summer holidays (at least from the northern hemisphere perspective). Team members are scattered on vacations, VCs are out of the office and lawyers are.. well.. lawyers will always be there. So it's understandable why there's a lack of officially founded companies during the summer. However, that doesn't mean great companies do not form during the summer.
I understand that many of these companies had their original idea brewing for some time and they likely launched it prior to incorporating their company, so the founding date is more of a formality rather than the date of idea inception.
There is no rule that you cannot start your company in the summer. Maybe it's the best time since many of you will be out of school and have more free time. The boss may be out of the office so you can hack away in your cubicle. Yeah, you'll have to stay focused and not go to the beach as often, but for most entrepreneurs this isn't a problem. Those that are focused on the goal will reach it no matter what obstacles get in their way.
My suggestion is to start building something and put it in front of your target audience. Get feedback and react to them quickly. Start refining your product and practice with different revenue models to see what will hook on with the customers. Before you know it, you may have the hottest companies to ever be founded during the summer months.