Yes, having a great book is important. Unfortunately, it's not enough.
The reality is if you release a book and do nothing for it because you think people will buy it because it's amazing then, more than likely, no one will buy it.
How do you determine the right time to release your book? Here are some things I learned in my publishing classes. First of all, publishers release books on a Tuesday. I don't know why. If you want your book to have a professional feel about it, then consider releasing it on a Tuesday as well.
January (beginning of the year)- Good for self-help books. At the first of the year, people are making resolutions. Any book that might help readers make and keep resolutions would be perfect for this time of year. Also note, this is the back end of Thanksgiving, Christmas and the New Year. People's funds are limited. This is a slow month in terms of book sales.
February- Awesome for romance novels or books that can be Valentine's Day gifts.
March-May- You want your book to be a summer read, but you don't want to compete with every other author who wants to be on people's summer reading list. These months are for you.
April- This is National Poetry Month- ideal for releasing books of poetry.
June and July- Summer reads. This is when the big publishers release books... heavily. A good time for books people would want to bring with them on vacation. More than likely, this is not the best time to release a book with a heavy subject matter or your thousand page book that's not travel friendly, unless it's a ebook. It can also be tough for little known self-published books to get any sales during this time because of the competition from big publishers.
August- Publishers don't normally release books by big name authors during this month. New or little known authors can publish books in August and have a better chance at seeing some descent sales.
August-September- Back to school. Great time for textbooks and learning materials for school aged children.
Beginning of Fall- This is another period when big publisher are releasing their books. This may not be the best time for an indie author to debut. Like summer, fall is great for book sales in general, but you'll be going against some big name authors.
October- The month of horror. Great for horror, paranormal and gothic books. Remember, you will not be the only one who thinks this. October will be flooded with horror-related books.
November-December- This is hit or miss. People will be focused on Christmas presents. Now, if your book is Christmas themed or you can convince people it's a good gift, then this is your season. This would also be a good time for hardcover books.
December-January- Good for ebooks. They cost less than prints books which is awesome for people who spent much of their money on Christmas presents. Also, a lot of people get e-readers and tablets for Christmas. They'll be looking for ebooks to put on their new devices.
Think about themes in your book and see if you can tie it to a holiday or an awareness month.
This is not about when is the best time to promote your book. That's 24/7. This is thinking about when readers buy books and when big name authors release their books. You may not want your horror novel released at the same time as the new Stephen King title.
That being said, for some authors, it doesn't matter when they release their book. People will buy it anyway. Most of us first time authors haven't reached that status.
You may disagree with the way traditional publishers run things, but their job is to sell books. And, you have to admit, they do it well. They spend a lot of time deciding when to release a book. The release date must be important.
Picking an ideal date, won't guarantee that your book will be a hit, but it will help your sales. Remember the four P's. The release date is only part of the equation.