This song is about the personality and way of loving often associated with people born on Monday. I think it describes them quite well, since I was born on a Monday.
Translated Lyrics(part1)
Because I was born on a Monday, the way I love is deep and enduring.
I love you quietly, without grand gestures, yet with a bond so strong that I could never forget.
I may seem calm and reserved. I may not tell you every day that I miss you, but my heart loves you more intensely than the moonlight shining on a beautiful night.
My love is sincere and honest. I have never wanted to deceive anyone.