### Instruction: Lucy was born on Wednesday, December 1st, 2004. That Wednesday was the first day of her life. Her parents held a party for her on the $1000$th day of her life. On which day of the week was the party?### Response: To find the $1000$th day of Lucy's life, I need to add $999$ days to the day she was born, since the first day counts as one.
One way to do this is to use modular arithmetic, which deals with remainders after division.
For example, if I divide $999$ by $7$, I get a quotient of $142$ and a remainder of $5$.
This means that $999$ is $5$ more than a multiple of $7$, so adding $999$ days to a Wednesday is the same as adding $5$ days to a Wednesday.
To add $5$ days to a Wednesday, I can count forward: Thursday, Friday, Saturday, Sunday, Monday.
So the $1000$th day of Lucy's life is a Monday.
Another way to do this is to use a calendar and count the days one by one, but that would take a lot longer and be more prone to error.