Otter Day: Weekday Guesser - a learning game for IOS and Android
What you learn
Gameplay Impressions

Overview

Created as an independent hobby project out of a passion for mathematics education, Otter Day was submitted for the RevenueCat Peace Prize, as well as the Best Vibes Award and the Design Award. The Peace Prize is the main focus because the project grew out of an idealistic motivation: making mathematical thinking more accessible, playful, and memorable through a small, independently developed learning experience.

Inspiration

As a teenager, I read Secrets of Mental Math: The Mathemagician's Guide to Lightning Calculation and Amazing Mental Math Tricks by Arthur Benjamin. The final chapter introduced a method for determining the day of the week for any date in seconds — entirely in your head.

I was blown away. How could such a powerful trick be so simple, yet so little-known? I couldn’t stop sharing it with everyone I met.

After high school, I faced a tough decision: Psychology? Pedagogy? Math?
I ultimately chose math — and looking back, I’m convinced this trick played a big role. It gave me a glimpse into the “magic world of patterns and numbers” and, at a formative age, gave me an arguably impressive skill.

Fast forward 10 years: I’m revisiting this trick because I still believe it deserves more attention.
For me, this project is a perfect full-circle moment — combining psychology, pedagogy, and math into one fun learning experience.

What It Does

At its heart, Otter Day is a small piece of math education wrapped in a game. It teaches you to instantly determine the day of the week for any date — with rich storytelling and motivational elements designed to keep you improving.

It’s perfect for anyone with the “Sudoku gene,” number enthusiasts, or anyone who just wants to learn a neat party trick.

Features include:

Immersive Tutorial – Learn in a steampunk classroom, sitting alongside Liv (a clever otter) and Trevor (a hilariously clueless otter). The Otter Teacher guides you through a four-part tutorial that makes the method easy to grasp.
Two Game Modes:
- Text Mode – See the date on screen.
- Audio Mode – Hear the date spoken aloud, just like real life when someone casually mentions a date.
Six Levels of Difficulty:
- from Level 1: Dates from 2025 (easy)
- to Level 6: Dates from 1701–2099 (hardest)
Bonus Level: Choose a fully custom date range (1700–2800).
Achievements & Rewards: Earn three types of chronoshards:
- Precision – Rewarded for accuracy.
- Speed – Rewarded for faster answers.
- Marathon – Rewarded for consistent practice.

How I Built It

Otter Day was built as a solo hobby project — late nights, long weekends, and lots of support from friends and the build-in-public community who provided regular feedback.

Inspirations came from Duolingo and Sudoku games.

Today, I can confidently solve dates in under 5 seconds — but the real goal is to make this app fun, accessible, and motivating for anyone who wants to learn it. Since launch, I’ve already improved the app with:

A step-back button in the tutorial
A completely revamped month tutorial

I relied on multiple AI tools to bring it to life:

Perplexity Pro – For refining dialogues, generating characters, icons, mnemonic visuals, and sentences
KlingAI – For animating otter characters in a Warcraft/Starcraft style
ElevenLabs – For text-to-speech, giving Otter Liv her voice

The app was built with Flutter, allowing me to ship simultaneously to iOS and Android.

Also, I used Shorebird for code push.

Challenges

Creating Otter Day wasn’t without hurdles:

Designing for Different Learners: Understanding how different people learn and what helps them stay focused and motivated.
Supporting Different Learning Styles: Catering to visual, auditory, and reading/writing learners — each with different preferences.
Balancing Minimalism & Storytelling: Too many animations can distract — this can be particularly detrimental in this app, as you need to focus your mind; on the other hand, too few make the experience dull.
Designing the Narrative: Choosing characters and settings that make learning fun without overwhelming players.
Clarity: Explaining the method with as few words as possible so players can quickly jump into “learning by doing.”

Accomplishments

Successfully designed a game that supports all learning styles:
- Visual: Pattern-based month codes and memo-friendly illustrations
- Reading/Writing: Mnemonics and clear step-by-step instructions
- Auditory: Spoken mnemonics by Liv the otter
Iterated Through Feedback: Continued improving the app based on feedback from players and the build-in-public community.
Received lots of praise for the visuals from players and even Flutter community leaders (Mike Rydstrom and Filip Hráček)
Nailed the steampunk aesthetic
Built what is likely the most comprehensive app for learning this trick
Created something that I’m genuinely proud of — and enjoy using myself

Lessons Learned

Learned the intricacies of deployment, from app stores to updates
Explored how AI can be used for rapid prototyping and creativity (images, voices, dialogue)

What’s Next

Feature Development: Upcoming features may include:
- Streak tracking & home widget (like Duolingo)
- History trivia mode
- Hands-free mode for people with disabilities
- Speed and accuracy progress stats
- Social features like a “versus mode” or leaderboards