What is the Day of the Week Calculator?
This tool tells you which day of the week any calendar date falls on — past, present or future. Whether you want to know what day you were born, when a historical event happened, or which weekday a future deadline lands on, enter the year, month and day and get the answer instantly. The calculation uses Zeller's congruence on the proleptic Gregorian calendar.
How to use it
Choose the year, pick the month from the dropdown, and enter the day of the month. Press calculate and the result box shows the weekday name. The table also reports the raw Zeller index (h) and the ISO weekday number (1 = Monday … 7 = Sunday) for programmers and the curious.
The formula explained
Zeller's congruence computes the weekday with pure arithmetic — no lookup tables. January and February are treated as months 13 and 14 of the previous year, which keeps leap-day handling simple. With q = day, m = adjusted month, K = Y \bmod 100 and J = the century (year ÷ 100), the formula is:
Here h = 0 is Saturday, 1 is Sunday, and so on up to 6 = Friday.
Worked example
For 13 March 2020: q = 13, m = 3, y = 2020, so K = 20 and J = 20. Then
使用注意事項與精準校對原則（6 大核心守則）
📐 1. 嚴格對齊輸入量綱與單位
輸入各項數值時請務必確認量綱（例如小時、天數、金額、百分比），避免單位不匹配引發推算偏差。
🔒 2. 純前端本機毫秒級隱私保障
所有計算邏輯 100% 在您的終端瀏覽器內完成，資料絕不上傳伺服器，嚴格守護商業與個人隱私。
📊 3. 符合公認學術與產業計算模型
演算法嚴格遵循國際通用數學公式與標準計算模型，經多重情境交叉核對，數值嚴密精確。
📋 4. 一鍵複製與免登入 PDF 匯出
支援一鍵複製推算結果字串至剪貼簿，亦可直接點擊列印按鈕輸出純淨 PDF 工作報表。
🔄 5. 多情景敏感度動態對比
建議針對極端值與多組預期情景分別試算，觀察輸出數據的波動區間，輔助做出更穩健的決策。
🛡️ 6. 專業諮詢與日常參考免責聲明
本工具運算結果供日常排程、學業研習與事前參考之用，如涉及重大法規或投資決策請諮詢持證專家。