Don't pretend just because of a holiday or whatever, that's silly and insulting to both you and her. You aren't married or anything incredibly serious or you wouldn't feel comfortable breaking up over the phone.

Honestly if it is logistically impossible to do it, then breaking up over the phone is acceptable, but only if you can't see each other in person. Just don't do it over msn or some other text based medium because that's just going too far. I've broken up over the phone though and it worked out fine, we stayed friends right away, it just wasn't possible for me to see her in-person to do it, but it had to be done for both our sakes. Just don't make a habit out of it, this is a last option.

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"The 'Brick Walls' are there to allow you to prove how badly you want something!" ~ Randy Pausch

~ Rye