In US English, one would never use 'festival' for Easter or Christmas or anything. It would be a 'holiday'. Festival in English is more like a music fest, where a lot of bands/singers are performing, or a big gathering where there might be a lot of things to do/see (like a 'brat fest' where people get together and eat German foods and the like). Usually these things are outdoors and people wander around, eating/listening to music/watching people present things.
'Det' here doesn't refer to 'fest', though. For 'fest' you've used 'en', which means it's not neuter. 'Det' is used in the same way as the English 'it' in this sentence, in a more general sense referring to the whole sentence or the whole phenomenon you're describing, not to a specific grammatical word.