Looking to complete the main quest? You're just in luck! Here's a simple guide on how to complete as well as some images to help you find what you're looking for. Links to images will be mostly placed at the end of a step description.

If you would prefer to see a video version of this guide, there is a link to MJPW's video at the at the end of bottom of the guide.

Step 1: Turn on Power
It's as simple as that! Whichever way you choose to go to power doesn't matter, but you will eventually need every area inside the building to be accessible because everything for this main quest is inside the building.

NOTE: Steps 2 through 5 can be done in ANY order.

Step 2: Collect the Balloon Boy Poster
Inside the main building near the arcade cabinets there is a poster/drawing of Balloon Boy. Simply interact with it to collect it and there will be an audio cue letting you know you've done the step correctly.
https://steamcommunity.com/sharedfiles/filedetails/?id=2879827629

Step 3: Arcade "Button Masher"
In the arcade area, there are two cabinets that have blue buttons on them. All you need to do is shoot the buttons with an upgraded weapon in the following order:

Top Left. Bottom Left. Top Right. Bottom Right.

Once done, there will be the bomb defusal sound from Gorod Krovi to let you know you did the step correctly. I you hear a buzzer instead, you did it wrong and will have to wait until the next round to try again.
https://steamcommunity.com/sharedfiles/filedetails/?id=2879827794

Step 4: Cupcake Quest
Around the inside of the building are 3 cupcakes. All 3 need to be shot with an upgraded weapon. There will not be an audio cue letting you know that you've completed this step. You can find the locations in the following spots:

On a table next to the Foxy mask and a box location: https://steamcommunity.com/sharedfiles/filedetails/?id=2879824061

On the floor next to a perk location at the other end of the hall from the Foxy: https://steamcommunity.com/sharedfiles/filedetails/?id=2879824152

On top of an arcade cabinet near the power room: https://steamcommunity.com/sharedfiles/filedetails/?id=2879827299

Step 5: Spawn Wall Code
In the spawn room there are wall tiles that act as a number pad. There is an orange-ish red discoloring tile to let you know that the tile beneath it is the number 2. You will need to shoot the wall tiles with an upgraded weapon to input the code.

The code for this wall is: 395248.

Once done, there will be the bomb defusal sound from Gorod Krovi to let you know you did the step correctly. I you hear a buzzer instead, you did it wrong and will have to wait until the next round to try again.
https://steamcommunity.com/sharedfiles/filedetails/?id=2879823052

Step 6: Play FNAF 2
After completing the aforementioned steps, the arcade cabinet with a pumpkin floating over it should now be on with an interact prompt to "play FNAF 2."
https://steamcommunity.com/sharedfiles/filedetails/?id=2879839786

Once you interact with the machine, you will be teleported into a small recreation to the office from FNAF 2.
https://steamcommunity.com/sharedfiles/filedetails/?id=2879840642

Your goal is to survive a wave of constant spawning zombies in this room. If you manage to survive long enough, you will be teleported back to the main map and have an 8-bit Freddy mask above your points in the HUD letting you know you've completed the "Happiest Day" quest!
https://steamcommunity.com/sharedfiles/filedetails/?id=2879614469

if you found the above guide a bit confusing, check out MJPW's video guide here: https://www.youtube.com/watch?v=1kl1N1IQ0kw