A holiday custom comprised of artificial origins, is perhaps the most genuine of all, in a religion full of holiday nuances, usually introduced in the Books of Moses, and detailed extensively in one of the two Talmuds.   The very common custom that defies those beginnings is called tashlikh, "to send" and occurs on the Jewish new year, to start ten days of repentance.

The ten days of repentance, as I know them, only involves a line of prayer added to each of the three daily prayers, but I'm not an expert.   Also, I rarely say prayers more than twice a day, but the two for day and night are important to me.

Tashlich involves a group of prayers walking to a moving body of water, plucking some grass or leaves, and tossing them in, signifying the taking away of sin.   I recently saw that pebbles or stones are used, which I would imagine is for arid regions.   The same source said bread is thrown in, but that breaks the roots of pure nature in the tradition.

After the 'ten days' are complete, the next day is the holiday of Yom Kippur, meaning 'day of repentance' where a 24-26 hour fast is invoked.

Four days after that, the eight day holiday of building and eating in a hut begins, called Sukkot and sukkah respectively.   The hut's build is special, as the roof must be of whole, organic origin, and a certain percentage of stars and night sky must be visible looking up, which inspires me profoundly.   There are other necessities described in one or another of the Talmuds or commentaries, which are impractical to the point of being comical.

Be prepared to learn a few more holidays, and many details, or depending on what strictness your process involves, many more significant days.