It is all right with you if I postpone our meeting/date/appointment/dinner/whatever until *xyz*/reschedule our [whatever] for *xyz*? My sister's birthday is on *xyz*. If so, I could stay (for) longer.

'Is it ... ?' . . .

Would it be ok with you if we postponed our get together to February 15, since February 10 is my sister's birthday?

Das ist der erste Teil. Worauf bezieht sich das längere Bleiben, auf die Geburtstagsfeier der Schwester oder auf das hinausgeschobene Treffen?

Sorry! That's what I meant, of course.

Es bezieht sich auf das Treffen =)

Well, and what kind of Treffen is it?

The occasion, please, and whether it's business, formal, a casual acquaintance, etc.

Our first date ;)

Oh, then take Werner's "get-together" (but with hyphen, please). That's fairly innocuous for someone you don't yet know personally.

And for the second part, "If so, I would have more time free" would also be okay.

You might also be able to say "meeting" in this particular context, but I'm not sure. I've never met up with someone (not on business) this way.

Thank you very much ;)What would I do without you? ;)