Since 2012, Apple has unveiled its latest iPhone in September. Typically, the new iPhone is available to preorder a few days after Apple's announcement.
Here's the recent history:
2012: iPhone 5 was announced on Wednesday, September 12, and started shipping on Friday, September 21.
2013: iPhone 5S was announced on Wednesday, September 10, and started shipping on Friday, September 20.
2014: iPhone 6 was announced on Tuesday, September 9, and started shipping on Friday, September 19.
2015: iPhone 6S was announced on Wednesday, September 9, and started shipping on Friday, September 25.
2016: iPhone 7 was announced on Wednesday, September 7, and started shipping on September 16.
2017: iPhone 8 and iPhone X were announced together on Tuesday, September 12. The iPhone 8 started shipping on September 22. In a change from previous years, iPhone X started shipping on November 3, almost two months after the announcement.