What comes around every 28 years?

I bought a 2001 Lonely Planet desk diary in 2002 or perhaps late 2001 in a remainder sale. It was for the pretty pictures from all over the world. Unfortunately it's a heavy affair so I should use it up or dispose of it.

2018 had the same calendar as 2001 but I missed that opportunity. 🤔 Next chance is 2029. Not surprising as calendars repeat every 28 years (LCM of 4 and 7), if you ignore the 100 year and 400 year rules. I may not know what a calendar is by then.

But then I never did use desk diaries so I should just flip through one picture a day and then discard the diary. 😐