I'm having the Monday blues.
→  월요병인가 봐. 

[예문]

A: Whew..I can't concentrate today.
    I'm having the Monday blues.
A: 휴... 오늘 집중이 잘 안 되네. 월요병인가 봐. 

B: Yep, it's not easy to start out a Monday.        
B: 응, 월요일은 원래 일이 손에 잘 안 잡히는 날이야.

A: What did you do during the weekend?      
A: 주말에는 뭐 했어?

B: I just relaxed and took a lot of rest.
B: 그냥 편안하게 푹 쉬었지.