The event begins at October 1st to October 8th and again from October 17th to October 24th.
Your world will be successful as soon as a total of 40 loads (4000 Bunches of Winterberries) are stomped.
The reward of the event will start at the next server save right after completing the stomping of 4000 Bunches of Winterberries.
You have from October 1 to October 8 to complete it in the first batch, then you have from October 17th to October 24th to complete it again in the second batch. You can benefit from a total of 14 days of enhanced potions if you succeed on both events.
You can complete both events in the first day, then in the next server save the reward of enhanced potions will be already active, lasting 7 days, but keep in mind that you can only load 10 Bunches of Winterberries per character in 24 hours, so that means that completing it on the first day requires atleast 400 characters using 10 Bunch of Winterberries, making 40 runs and not failing. To make sure each load was successful, the message "Alright you did it! Good stompin' everyone!" will appear at the end. The event lasts for 7 days, so you can combine with your world to do some loads per day, avoiding the need of having 400 characters. Also keep in mind that you need to do atleast 6 loads per day to complete the 40 loads until the end of the event. Some Wikis says that are 5 loads per day, but I don't know if the 8th day counts as possible to do the load, so to be safe, consider 6 loads per day, counting 42 loads in 7 days (you will obviously do only successful 40 loads).
After completing the event, the reward of 7 days with enhanced effect on potions will start right away at the next server save. You can complete both events delivering 8000 Bunches of Winterberries (80 loads) and benefit from a total of 14 days with 33% effectiveness on Health and Mana potions. Also, if you contribute for 3 years at least one Bunch of Winterberries for one of the events each year, you will receive the achievement Berry Picker.
Source: TibiaFandom and personal experience with the event.