On which day of the week did the date 25th January 1981 fall according to the Gregorian calendar?

Introduction / Context:
This problem asks for the day of the week corresponding to a specific historical date, 25th January 1981. Such questions help you practise mapping a date to its weekday using systematic calendar techniques or known references.

Given Data / Assumptions:

• Date: 25th January 1981.
• We use the standard Gregorian calendar.
• No additional reference weekday is given, so we either apply a formal algorithm or rely on known calendar anchors.

Concept / Approach:
There are several standard ways to determine the day of the week for any given date:

• Use a known reference date in a nearby year whose weekday is known.
• Apply an odd-days method, combining contributions from the year, month, and day parts.
• Use a formula-based approach such as Zeller's congruence or similar.

All these methods reduce the problem to computing a certain number of days offset from a known weekday and then taking the result modulo 7.

Step-by-Step Solution:
Step 1: A reliable reference point is that 1st January 1981 was a Thursday. Step 2: Count the days from 1st January 1981 to 25th January 1981. Step 3: From 1st January to 25th January inclusive contains 25 days, so there are 24 days between the dates. Step 4: Compute 24 mod 7. Since 7 * 3 = 21, the remainder is 3 days. Step 5: Move 3 days forward from Thursday: Thursday → Friday (1), Saturday (2), Sunday (3). Step 6: Therefore, 25th January 1981 falls on a Sunday.

Verification / Alternative check:
Using a perpetual calendar, a date-calculation program, or any reliable online resource, you will find that 25th January 1981 is indeed a Sunday. This serves as a strong confirmation of our manual weekday computation. Multiple independent tools agree on Sunday as the correct weekday.

Why Other Options Are Wrong:
Monday: One day after Sunday, not consistent with the computed 3-day shift from Thursday.

Friday: Two days before Sunday and only one day after Thursday; this would match 2 days instead of 3.

Tuesday: Three days after Saturday or four days after Friday; the overall offset does not match the 24-day difference from the start of January.

Saturday: One day before Sunday and two days after Thursday; again, not consistent with the exact remainder of 3.

Common Pitfalls:
Typical errors include miscounting days within the month (for example, mistakenly using 25 instead of 24 as the number of days between the dates) or misremembering the weekday on 1st January. Additionally, candidates sometimes move backward instead of forward or lose track when adding days. Writing out the sequence of weekdays clearly and carefully applying the modulo 7 remainder helps avoid such mistakes.

Final Answer:
25th January 1981 fell on a Sunday.