"The most loneliest day of my life" não é lá muito correcto no que diz respeito à gramática... mas na minha opinião o Daron Malakian queria enfatizar que aquele dia foi, de facto, extremamente solitário (Não me refiro ao jogo de cartas, mas ao outro solitário... quer dizer, pensando bem...).

Lonely day - System of a Down

Such a lonely day
And it's mine
The most loneliest day in my life

Such a lonely day
Should be banned
It's a day that I can't stand

The most loneliest day of my life
The most loneliest day of my life

Such a lonely day
Shouldn't exist
It's a day that I'll never miss

Such a lonely day
And it's mine
The most loneliest day of my life

And if you go,
I wanna go with you
And if you die,
I wanna die with you
Take your hand and walk away

The most loneliest day of my life
The most loneliest day of my life
The most loneliest day of my life

Such a lonely day
And it's mine
It's a day that I'm glad I survived