How are the enemy levels determined? are they determined by the highest lvl pet in our party or by our character level?

Depends on the zone and the event. Its usually decided by the highest level pet in your party. Some zones, like the LG, dont scale levels and have everybody fightin the same level enemies.

What about the Summer Festival?

Events change every year. Were usually told in the fest overview whether or not levels are scaled.

That said, season fests typically have their own zone and will level scale. Youll still want to check the Iverview once it is released, though.

I have never seen a Seasonal Festival not level scale.

The Seasonal Festivals, which take place in Fest Zones, are always scaled to your highest party level (previously with some fluctuation to about five levels under, if you were level 60, but as of the release of the Lost Grove, all enemies match your highest pet).
The sole exception being if you are under level 10- no enemies will ever be under level 10.

'Event' Festival enemies- such as the Fortune Festival Pookas- which do not have their own zone keep to the levels of the area you are in (eg. lv70 for stage 6 Lost Grove, lv50-60 for Astryl's Tranquility)

Ok thanks!