The calender is based on the seasons, and each is usually broken into 3 months. The seasons are based on solstices and equinox, and thus are usually identical across the world.
The months themselves are somewhat different, as the naming scheme is wholly dependant on the nation setting the calender. Some are based on the birthdays of great kings and heroes (for example “Herstus’s Month), cultural traditions, or any number of others. A great deal can usually be gleaned as to what a people find most important just by asking the date.