The setting is that there is a celebration underway where people of different species (lets say, humans and elves) are attending. In the elves' language they call any celebration "Jashnak". So, if they are welcoming someone they would say "Welcome to the Jashnak!" but then I am not sure how subsequent references should be handled ?
For e.g. In case of a third party observer
The lanterns hovered above the marquee where the Jashnak was underway.
In case of two humans talking
The Jashnak sure is getting rowdy!
OR
Come on... Get ready quickly. We have a Jashnak to attend.
If I was to use "celebration" instead of "Jashnak", the celebration in the two examples would not be capitalised (on account that it is a common noun).