Downloads - Day Finder

## Program Description

The **Day_Find** program, alongside its core mathematical helper function **day**, is an interactive calendar calculation utility written in Casio Basic for the Casio fx-CP400 and fx-CG500 graphing calculators. The program functions as a dedicated "Day Finder" designed to determine the exact day of the week for any given calendar date in the current era.

When executed, the program processes three inputs: the date (k ) , the month (m), and the full year. It automatically extracts the century (c) and the specific year of that century (D) to prepare for chronological math. Before running the core calculation, **Day_Find** performs strict error-checking to ensure that all values fall within realistic calendar boundaries. It cross-references month lengths (30 or 31 days) and incorporates comprehensive leap year algorithms to validate February dates. To execute the day-matching logic, it normalizes January and February as the 11th and 12th months of the *preceding* year to comply with Zeller's Congruence.

The adjusted parameters are then passed to the **day** function, which runs the modular formula:

$$\text{mod}\left(k + \lfloor\frac{13m-1}{5}\rfloor + D + \lfloor\frac{D}{4}\rfloor + \lfloor\frac{C}{4}\rfloor - 2C, 7\right)$$

The function returns an integer from 0 to 6, which the main program maps directly to a popup message displaying the matching day from Sunday to Saturday.
---

### How to Use
*
**Run with Arguments:** Execute the program from the Program application or the Main application screen by passing the date, month, and year as arguments: `Day_Find(Date, Month, Year)`.
*
**Input Validation:** * Ensure your date (k) matches the real limits of that specific month (e.g., trying to input a date greater than 30 for April or 31 for January will throw an appropriate error message).
* Enter the year as a full 4-digit integer (e.g., 2026).
*
**View the Result:** If the inputs pass all calendar validation checks, a dialog box titled "Day Finder" will immediately appear on the screen, displaying the calculated day of the week (e.g., "Thursday").
---

### Precautions for Placing the `day` Function in a Folder

Because the main **Day_Find** script references an external custom function block (`day(k,m,C,D)`), proper directory path management is necessary:

* **Default Directory Pairing:** Casio Basic searches for custom functions within the active folder directory (usually the `main` folder) unless told otherwise. To prevent directory communication errors, keep both `Day_FinF` and `day` in the exact same folder. OR, I'D SUGGEST KEEPING THE 'day' FUNCTION IN THE LIBRARY FOLDER FOR SEAMLESS ACCESS.


* **The Folder Path Breakage:** If you isolate the `day` function inside a separate subfolder (such as an organizational folder named `CALC`(except for the library folder) ), the program will crash with an "Undefined variable or function" error because it cannot find the file in the local root.
---

This programme is yet under development, and any improvements, suggestions, comments, bug reports, are always welcome, and may be communicated to me!

### Disclaimer

> **Notice:** This calculator program is provided "as-is" for educational, hobbyist, and quick reference purposes on supported Casio ClassPad calculator platforms. While it utilizes precise modular congruence arithmetic to calculate accurate day names, it should not replace official calendar software for critical legal, historical, or professional scheduling. Always cross-check system parameters if an unexpected interruption occurs.