I'm not sure I fully follow your question.

The default calendar in Project (Standard calendar) has all Saturdays as non-working. You can create a new calendar with every other Saturday as working and then copy that calendar to the Global template, making it available in all new projects.

So, is your question

• How to create a calendar with every other Saturday as working time?
• How to copy that calendar to Global?
• Something else?

Please also let us know what version of Project. The directions for changing calendars may be different depending upon what version of Project you use.