It was Monday on January 4, 2021. What will be the day

Solution

Number of odd days in normal year is 1 and number of odd days in leap year is 2.
There are 4 normal years (2022, 2023, 2025 and 2026) and 1 leap year 2024 from January 4, 2022 to January 4, 2026.
So, number of odd days will be
From January 4, 2021 to January 3, 2022: 1
From January 4, 2022 to January 3, 2023: 1
From January 4, 2023 to January 3, 2024: 1
From January 4, 2024 to January 3, 2025: 2
From January 4, 2025 to January 3, 2026: 1
So, total number of odd days is 6. So, on January 4, 2026, it will be Sunday (As Monday + 6 odd day = Sunday)