There are some basic settings that apply to all calendar objects. You can change these settings using the Posterino preferences or the System Preferences of your Mac.
To change calendar preferences:
Open the Posterino Preferences window (Command-',') and display the General panel:
Under Calendars you can change the following settings:
The default year of a newly created calendar object. Smart means: If the current date is between January and September Posterino will use the current year as default and if the current date is between October and December Posterino will use the next year as default.
The days considered as weekend and thereby emphasized differently than the other days of the week.
Calendar properties like calendar system (Gregorian, Hebrew, etc.) or first day of week can be controlled by the Language & Region panel of the System Preferences of macOS:
Please also note, that the language of the calendar object is controlled by the topmost item of the "Preferred Languages" list on the Language & Region panel shown above.
You can also change the language of the calendar object using the Language property on the Year calendar inspector panel.