Kwan reads a book that is 400 pages long at a rate of 1 page per minute. He reads for half an hour each day, which equals 30 minutes per day.

### Step 1: Calculate the number of pages Kwan reads per day

Since Kwan reads 1 page per minute and reads for 30 minutes each day:

\[

\text{Pages per day} = 30 \times 1 = 30 \text{ pages}

\]

### Step 2: Calculate the number of days required to finish the book

To find the number of days required to finish the book, divide the total number of pages by the number of pages he reads per day:

\[

\text{Number of days} = \frac{400 \text{ pages}}{30 \text{ pages/day}} = \frac{400}{30} \approx 13.33 \text{ days}

\]

Since Kwan cannot finish the book in a fraction of a day, we round up to the nearest whole number:

\[

\text{Number of days} = 14 \text{ days}

\]

### Step 3: Determine the day of the week

Kwan begins reading on a Saturday. We need to find out what day it will be 14 days later.

Starting from Saturday:

- 7 days later: Saturday (end of 1st week)

- 14 days later: Saturday (end of 2nd week)

Thus, Kwan will finish the book on a **Saturday**.

### Final Answer:

Kwan will finish the book on \( \boxed{\text{Saturday}} \).