This is a simple quest which can be completed after you’ve returned from the Shattered Library during the Trespasser main quest. At this point, go to the south western section of The Winter Palace courtyard, as shown below, to find a number of fireworks. This can be set off and then viewed through the nearby telescope.
You can acquire some rewards if you detonate the fireworks at their highest possible point. These rewards are as follows:
+1 Magic stat (earned with 50 - 65 points)
+3 Magic stat and a Sigil of the Golem (earned with 65 - 80 points)
+6 Magic stat and a Chromatic Greatsword Schematic (earned with 80 - 100 points)
+10 Willpower stat (earned with 100+ points)