There are planty of calendar apps out there. Sunbird/lightning (mozillia), rainlendar, etc.
If you already use an existing calendar program, you can always add another category to it then be able to show/hide it. For example I use google calendar now since it syncs with my phone and have my calendar with all my work/holiday/vacation days and anothr calendar for each of my kids showing their school days and days they are off school. This way I can look and coordinate vacation days around holidays or days the kids are off or plan larger project work around days where everyone has a full 5 day week. When/if I ever fo gat a hunting license again I'll just add that schedule as an additional in another color so I can coordinate as well.