Answer:
Thursday.
Explanation:
We have that the first day of Alex's trip was a Monday, that is to say that the next Monday will be obtained by adding 7 to the previous numbers.
1 + 7 = 8
8 + 7 = 15
15 + 7 = 22
22 + 7 = 29
29 + 7 = 36
We have that the 36th is a Monday, therefore:
37, Tuesday
38, Wednesday
39, Thursday
In other words, the 39th would be Thursday.