OK, thanks a lot !They are not interchangeable, but their meaning overlap partially.
The core meaning of 'selskap' is 'company', and this includes a trading company and a social company/gathering. A gathering can be also a feast, hence in such occasion the meaning overlaps with feast (fest).
'Fest' means only 'feast'.
Mine venner har selskap på lørdag.
Mine venner har festen på lørdag.
I can add that although both "selskap" and "fest" can be translated as "party", there is a nuance: "selskap" has a more limited use, and is mostly used for more formal occasions.
If it is an informal gathering, you should use "fest" - but "fest" can also be used in more formal contexts (for example "bryllupsfest", (wedding party)) .
These sentences are not quite comparable. The noun is in the indefinite form in the first sentence, and in the definite form in the second.