If 1st January 2020 is a Wednesday, then what will be the day of the week on 15th September 2020?

2020 is divisible by $4$, so it is a leap year and February has $29$ days.

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Days from 1st January to 15th September:

January (after the 1st) $= 30$

February to August $= 29 + 31 + 30 + 31 + 30 + 31 + 31 = 213$

September $= 15$

Total $= 30 + 213 + 15 = 258$ days

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Every $7$ days the weekday repeats, so only the remainder matters:

$258 = 7 \times 36 + 6$

Remainder $= 6$

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Moving $6$ days ahead of Wednesday:

Thursday, Friday, Saturday, Sunday, Monday, Tuesday

Therefore, 15th September 2020 is a Tuesday.