http://www.atlasobscura.com/articles/why-russian-astronauts-pee-on-a-bus-tire-before-launching-into-space-and-other-preflight-rituals
According to the ESA, Gagarin was on his way to the launch pad in 1961
when he realized he needed to urinate one last time. The bus was
stopped, and Gagarin got off, headed to the back-right tire, and
relieved himself. As a tribute, each bus trip to the Baikonur launch pad
now incorporates a stop, during which crew members pee on the
back-right bus tire.
There are quite a few "rituals" cosmonauts have to do. The tree planting, the door signing, the Russian Orthodox blessing...