Actually you need to complete the Manure Collection to get one Saltpeter. The collection is composed of the following:
Poultry Puddin’ – from Chickens and Geese
Piggy Poop – from Pigs
Brush Brownie – from Sheeps and Goats
Prairie Pile – from Mules and Horses
Meadow Muffin – from Cows and Oxen
You can get the manures from your animals or by tending your neighbors’ animals. Once you completed the Manure Collection, trade it in and you’ll receive the Saltpeter needed to craft Fireworks from your Cabin.
If you lack Fire, you can craft it inside your Wagon. You’ll need seven Woods and five Cloths to create two Fires. Completing the Fourth of July Part 1 Goal awards you 500 coins and 50 XP. You also have the chance to share some fireworks to your friends.
Have you completed the Fourth of July Goals? Among the four, getting the Saltpeter and the Ribeye (for Part 2) were the hardest.