It’s not a souvenir if you haven’t been there, it comes from the French for “memory”, by definition you can’t stay at home and buy a souvenir.

The solstices and equinoxes seem like a good natural choice. Various cultures past and present have celebrated them but since they’re natural events experienced by everyone they can be culturally neutral. They happen even independently of whatever calendar system you use and are important milestones of the year.