Your novel must open with bite.
One doesn’t hook a voracious reader, one creates a voracious reader by stimulating appetite.
I can tell you, as an avid reader, that I–so badly–want to be invited in. I crave an invitation that cannot be refused. That opening must build appetite and by appetite I mean, the desire to read –felt as hunger. Appetite exists in all bibliophiles, and serves to regulate adequate literary intake to maintain creativity, imagination, and contentment. Once you’ve stimulated that appetite, your reader wants to sit down at your table and devour your book. They’re hungry and they need to eat.
How do you stimulate appetite? Well, as a reader, I want — nay, I need something to grab my attention. If it’s properly presented the reader won’t just skim across it — their pupils should grow as they soak it in. You need a statement of unusual or vivid fact, a compelling question, a simple scene exhibited in a profound manner, an enticing or inciting mood setter.
Rather than using, Once upon a time, or It was a sunny day in July– take a look through American Book Review’s top 100 list to get some inspiration. http://americanbookreview.org/100bestlines.asp
Good luck, but know this– Once you’re happy with your opening, you will move on and through your story before coming back to the opening –of which you will undoubtedly change–and then again, three or sixteen more times.
Rest assured that once your opening line is solid, readers will notice, but –don’t leave it at that. Once the prey is drawn to the bait, you still need for them to bite, so make sure the whole opening scene is enticing. –I’m getting hungry just thinking about it.
You’ll know when you’ve got it right, but then –maybe your beta reader(s) will have a different idea. Either way, have fun! That’s why we read and write –for the pleasure, so enjoy it.