If it was a Monday on 1 January 2018, what was the day of the week on 29 December 2018.

Given that 1 January 2018 was a Monday, we need to find the day of the week on 29 December 2018.

The number of days from 1 January 2018 to 29 December 2018 needs to be calculated.

Since 2018 is not divisible by 4, it is not a leap year and has 365 days.

Days from 1 January to 29 December:

• January (after 1st): $31 - 1 = 30$ days
• February: $28$ days
• March: $31$ days
• April: $30$ days
• May: $31$ days
• June: $30$ days
• July: $31$ days
• August: $31$ days
• September: $30$ days
• October: $31$ days
• November: $30$ days
• December (up to 29th): $29$ days

Total days $= 30 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 29$

Total days $= 362$

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To find the day of the week, we calculate $362 \mod 7$:

$362 = 51 \times 7 + 5$

This means 362 days $\equiv 5$ days (mod 7)

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Starting from Monday (1 January 2018), adding 5 days:

Monday $\to$ Tuesday $\to$ Wednesday $\to$ Thursday $\to$ Friday $\to$ Saturday

Therefore, 29 December 2018 was a Saturday.

The correct answer is Option 1: Saturday.