How to make firework visible to others in my sanctuary?

Once you have started the event by going to the Holiday events list and activating the fireworks it will start to show the small icon next to your sanctuary in the world, you only need to do this once on the first day. After that as long as you have claimed the rewards and are under the max of 30 a day then others will see your fireworks box symbol.

I have noticed that the game doesn't always show all the available boxes all the time. But your fireworks will be visible to players who haven't yet helped you.