Honzík received a book for Christmas, which he started reading on Christmas Day. He read the same number of pages every day until January 31st of the new year. On that day, he found out that he had read 78 pages, which was exactly one-third of the book. At the same time, he realized that if he wanted to finish the book on the day of his birthday, he would have to start reading four more pages each day from tomorrow onwards.

Determine when Honzík's birthday is.
Since 78 pages correspond to a third of the book, the entire book has $3 \cdot 78=234$ pages. Honzík has $234-78=156$ pages left to read.

From Christmas to January 31, a total of $8+31=39$ days have passed. Therefore, Honzík read $78: 39=2$ pages per day. If starting from February 1 he reads four more pages per day than before, that is, six pages per day, he will finish the book in $156: 6=26$ days. Honzík's birthday is on February 26.

Evaluation. 1 point for each of the partial results (the book has 234 pages, 156 pages are left to read, he read for 39 days until the end of January, he read 2 pages per day, he will finish the book in 26 days); 1 point for the birthday date.