Learn Polish: A propos wakacji: dokąd jedziecie? - Speaking of holidays: where are you going?

Yes! Although it is borrowed from French, it is very commonly used in everyday Polish to mean "speaking of" or "by the way". When you use it to say "speaking of [something]", the noun that follows must take the genitive case.

Why is it wakacji rather than wakacje?

It takes the genitive case because it follows the phrase a propos. Because the Polish word for holidays, wakacje, is a plural-only noun, wakacji is its genitive plural form.

Could I use gdzie instead of dokąd here?

You will often hear native speakers say gdzie (where at) in casual speech, but dokąd (where to) is the grammatically correct choice. Because you are asking about a destination using a verb of motion, dokąd is exactly the right word.

Why use jedziecie rather than idziecie?

Polish strictly divides going on foot from going by a vehicle. Jedziecie (from jechać) means you are traveling by car, train, or plane for your holidays. If you used idziecie, it would imply you are literally walking to your holiday destination!