After talking to some friends who had no idea and just in case this reaches some players who may not be aware:
Super Mega Raids can only spawn at certain gyms so it's worth checking which of your local gyms can host them to avoid disappintment over Go Fest Global weekend.
If you go to the Map on the Pokemon Go website and log in you can view your area and check your gyms by clicking on them. It'll say "Super Mega Raid may appear!" on the ones which can.
Some of them will also have the Super Mega icon which makes it easier to check. However, some which don't have the icon can also host them. So click on each gym to double check.
by granty012