7. (10 points) Xiao Ming and Xiao Qiang often go to the library to read books. Xiao Ming goes to the library on the first Wednesday of January, and then every 4 days (i.e., the 2nd time is on a Monday). Xiao Qiang goes to the library on the first Thursday of January, and then every 3 days; if in January the two only went to the library together once, then this day is January  .

保留源文本的换行和格式，直接输出翻译结果如下：

7. (10 points) Xiao Ming and Xiao Qiang often go to the library to read books. Xiao Ming goes to the library on the first Wednesday of January, and then every 4 days (i.e., the 2nd time is on a Monday). Xiao Qiang goes to the library on the first Thursday of January, and then every 3 days; if in January the two only went to the library together once, then this day is January  .
7. (10 points) Xiao Ming and Xiao Qiang often go to the library to read books. Xiao Ming goes to the library on the first Wednesday of January, and then every 4 days (i.e., the second time is on Monday). Xiao Qiang goes to the library on the first Thursday of January, and then every 3 days. If the two only meet once in January, then this day is January $\qquad$.

【Solution】Solution: According to the problem:
If the first Wednesday and Thursday are in the same week, then the two will meet on the next Monday. They will meet again after an interval of $4 \times 5=20$ days, so the first day of January is a Thursday.

Then the dates Xiao Qiang goes are $1,5,9,13,17,21,25,29$.
The dates Xiao Ming goes are: $7,12,17,22,27$.
Therefore, the answer is: 17