Apple did something it had never done before: Released a version of macOS / OS X / Mac OS X on a Monday:

macOS 10.13 High Sierra: September 25, 2017 Monday

Mac OS X 10.0 Cheertah: March 24, 2001 Saturday
Mac OS X 10.1 Puma: September 25, 2001 Tuesday
Mac OS X 10.2 Jaguar August 23, 2002 Friday
Mac OS X 10.3 Panther October 24, 2003 Friday
Mac OS X 10.4 Tiger April 29, 2005 Friday
Mac OS X 10.5 Leopard October 26, 2007 Friday
Mac OS X 10.6 Snow Leopard August 28, 2009 Friday
Mac OS X 10.7 Lion: July 20, 2011 Wednesday
OS X 10.8 Mountain Lion July 25, 2012 Wednesday
OS X 10.9 Mavericks October 22, 2013 Tuesday
OS X 10.10 Yosemite October 16, 2014 Thursday
OS X 10.11 El Capitan September 30, 2015 Wednesday
macOS 10.12 Sierra: September 20, 2016 Tuesday