Hundred-year shift — If 1 January of year x is Monday, what is the first day of year x + 100?

Introduction / Context:
Advancing exactly 100 years changes the weekday by +5 (ordinary block) or +6 (if the terminal century year is a 400-multiple). We apply this to a Monday start.

Given Data / Assumptions:

• Year x starts on Monday (1 Jan).
• We seek the start day of year x+100.

Concept / Approach:
From Monday, add +5 → Saturday (non-400 century) or +6 → Sunday (400-multiple). Thus two possibilities exist.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with known sequences around 1900/2000 to see Saturday/Sunday outcomes.

Why Other Options Are Wrong:

• Friday/Friday or Saturday do not match the +5/+6 mapping from Monday.

Common Pitfalls:

• Assuming a single unique weekday when two are possible.

Final Answer:
Saturday or Sunday