Edit: The event is now over. I'm keeping this guide for future use in the next events, as I did with the egg hunt.

If you can't find a ball, I give some hints below and ultimately its location. They are enclosed in spoiler tags so that you can check each one individually without spoiling more than necessary. If you need an extra hint, post about it in the comments and I'll think of one for you (but try not to look at other people's comments as there may be spoilers!)

Also check out my other guide, Relationship Requirements Table (and much more).

This guide used to be about the Egg Hunt event, I have kept the info about it for curiosity/historical purposes.

Don't forget to like the guide so that it's visible for more people.

Hint: To find this ball, you must unclutter your vision.
Location spoiler: Poke any girl and click the eye icon to hide the dialogue pop-up

Hint: This missy, who's not aussie, but pretty bossy, wants to go see something sassy.
Location spoiler:Cassie's Movie Theater date image

Hint: This ball is so obvious you might as well be wearing it on your head...
Hint #2: Only the vain shall find this ball.
Location spoiler:Last item on the accessories for the player character in Statistics screen...

Hint: This ball is with a girl who will gladly play with your balls...
Hint #2: ... and you don't even need to buy her love first.
Location spoiler:Nutaku's Plushy Toy gift screen

Hint: You couldn't find this ball even if they made a it a law that you had to find it.
Location spoiler:Legal job animation

Hint: This ball is for those who have no attachment to anything...
Hint #2: ... you must consider some very radical ideas to find this ball.
Location spoiler:Full reset screen in "More..." menu

Hint: To get this ball, you must behave like an alpha male.
Location spoiler:Alpha's lover stage Seduce dialog box

Hint: Can't find this ball uh? That must be because you're tired... Try refreshing things.
Hint #3: All's fairy in love and war!
Location spoiler:On top of Q-Piddy's head in the login screen pop-up (you must close and re-open the game to get this one)

Hint: Everybody inquires on the whereabouts of this ball quite often.
Location spoiler:FAQ screen

Hint: This ball is with someone who's totally not based on a famous small equine.
Hint #2: Somewheeeeere over the rainbow, this ball flies... but wait, why is it all covered in sand?
Location spoiler:Iro's Beach date image (first hint is a reference to her hair being like Rainbow Dash's mane)

Hint #1: Only the chosen ones who can be born again are entitled to this ball.
Hint #2: The keeper of this ball is a certain bird of fate...
Location spoiler:The pigeon in Cassie's intro screen, you'll have to reset to get to it

Hint: Have you ever made someone so angry that they seemed to spit fire?
Hint #2: In Brazil we have a saying, "Don't poke a jaguar with a short stick"... well,
what's the closest thing to a jaguar you have in this game?
Location spoiler:Poke Bearverly when she has a low enough relationship level with you that she's pissed off by it. If she doesn't spit fire when you poke, reset

Hint: This ball is hiding in plain sight.
Hint #2: No, really, where's the place you'd least expect it to be?
Hint #3: Those greedy pandas! Always trying to tempt us into buying diamonds with discounts!
Location spoiler:In the Summer Event information screen ("20% off" sign)

Hint: One must give the pandas credit for hiding this ball so well...
Location spoiler:Second developer's face in Credits screen

Disclaimer: this event is long past, and this section is kept here mostly for sheets and giggles.

Hint: you must work really hard for this egg.

Location spoiler: Jobs screen

Hint: people frequently inquire on the whereabouts of this egg.

Location spoiler:In the FAQ screen in Settings

Hint: Oh, my memories of those times I loved the Moon...

Hint #2: What is the Moon called in other languages?

Location spoiler:Luna's lover picture in Memory Album. (Luna means Moon in Spanish).

Hint: This egg is forever.

Hint #2: You'd have to be very rich to obtain this egg.

Location spoiler:In the "More Diamonds" shop screen

Hint: You should skip on this egg, it's too hard to find!

Hint #2: You'll be looking for this egg for hours, so you might as well spare yourself the trouble...

Location spoiler:In the 4h time skip screen in the shop (users have pointed out it could be in any other of the buttons to buy things with diamonds. If it's not in the 4h time skip confirmation screen, try others!)

Hint: Finding this egg requires tweaking your way of thinking.

Location spoiler:In the Settings screen

Hint: Only those with time to spare can find this egg.

Hint #2: Finding this egg requires great courage and luck in the face of potentially mortal danger.

Location spoiler:Hobbies screen on top of Skydiving

Hint: Finding this egg is no walk in the park.

Hint #2: Finding this egg is so hard it requires two people.

Location spoiler:In the Moonlight Stroll date screen

Hint: Finding this egg requires an open mind and a willingness to redo your previous steps.

Hint #2: If you still cannot find this egg, sorry, you're too bad! You'd need to be born again with a smarter brain.

Location spoiler:In the Reset/prestige button screen

Hint: This egg probably has a honey filling.

Hint #2: Give up, finding this egg is bery hard.

Location spoiler:In Bearverly's stats screen

* Thanks to Finikksu for finding this egg and coming up with the first hint.

If you don't want the location spoiler but want to see more hints about a particular ball, comment about it and I'll think of a new hint for you.

Thanks to the user Cancer for providing the location of a few balls I couldn't find myself, and RemTM for a correction.