It references the foiled 5th November plot to blow up parliament. This is celebrated every year with bonfires and fireworks. They have a song/rhyme that starts "remember remember the 5th of November".

The picture is of a south park episode where they had "memberberries". These little berries would be nostalgic for old cultural references like the Goonies, or ghostbusters. They would communicate by asking if the others "membered" something and the others would all agree that they "member" that thing.