Jeffrey has been building a step by step series of tutorials which walks you through ever step for building your first Windows Phone Application.

As it is the 31st of October today, he's done.

You can find all the details here. Hopefully you can beat Jeffrey and complete yours in 20 days.

Day #1: Project Template
Day #2: Page Navigation
Day #3: The Back Button Paradigm
Day #4: Device Orientation
Day #5: System Theming
Day #6: Application Bar
Day #7: Launchers
Day #8: Choosers
Day #9: Debugger Tips
Day #10: Input Scope (on-screen Keyboard)

Day #11: Accelerometer
Day #12: Vibration Controller
Day #13: Location Services
Day #14: Tombstoning
Day #15: Isolated Storage
Day #16: Panorama Control
Day #17: Pivot Control
Day #18: WebBrowser Control
Day #19: Push Notification API
Day #20: Map Control

Day #21: Silverlight Toolkit
Day #22: Apps vs. Games
Day #23: Trial Versions of Your App
Day #24: Embedding Fonts
Day #25: Talking to Existing APIs (like Twitter)
Day #26: Sharing Your App With Other Developers
Day #27: Windows Phone Marketplace
Day #28: Advertising SDK
Day #29: Animations
Day #30: Gestures
Day #31: Charting