What is the ruling on saying "damn your day"?

It is not permissible to say "May your day be cursed" because it is a cursing of time, which is forbidden. The day and time have no power of their own; rather, cursing them reflects upon Allah Almighty, who disposes of all affairs. Therefore, one who curses time either curses Allah or associates partners with Him. Even if the word is a distorted form of "لعن" (cursing), its meaning and common usage indicate revilement. If a servant curses something that does not deserve to be cursed, the curse returns upon him. Furthermore, this phrase includes cursing a Muslim, and cursing a Muslim is an act of transgression (فسوق). It is incumbent upon a Muslim to guard his tongue.