You can check on the homepage if there is any active event. To access this menu, tap on the arrow you will find on the right bottom of your screen and then on the home icon.
Once you are in that menu, you will find the announcement indicating which event is currently going on and the corresponding countdown.
Clicking on the event’s announcement will transport you to the Portal location, though you can also find it on the quest map. If you are still far from the Portal, keep it up! Work your way up to that point and gain experience! Please remember that once the Event ends, its Portal will disappear from the map and all the respective Booster Packs will be removed from the shop.