Proto-Indo-European *sed- "to sit" became Vedic ni-ṣad "to sit or lie down" (ni- "down" from PIE *ni as in English beneath). This combined with upa "near to" to form उपनिषद् upaniṣád "to sit down near to; to approach, set about" hence "the sitting down at the feet of another to listen to his words" (Monier-Williams). The Upanishads are prose writings that are important to understanding the Vedas.
In Proto-Celtic, *sed- became the typically over-prefixed *eks-dī-sedo- (*eks- "out" and *dī- "out") "sitting". This became Welsh eistedd "to sit", which combined with bod "to be" to form eisteddfod "session". An Eisteddfod is a congress of Welsh bards.