Do people not have "Christmas Dinners" or "Easter Dinners" in Poland ?

I ask because my good wife tells me that it is tradition to have a large breakfast i.e. a cold, buffet style affair and anything that is left is brought out again later that day.

You obviously misunderstood your wife.
There is: Easter breakfast and Christmas Eve dinner.
Why? On Easter you celebrate the Resurrection of Christ. The lack of the body was noticed in the Sunday morning, that's why the breakfast is consumed.

It wouldn't be the first time and probably won't be the last ! :)

Let me elaborate. I am well aware that Christmas Eve "dinner" is Wigilia. I am referring to Christmas Day dinner and Easter Day dinner. She tells me Poles don't usually make a big occasion of Christmas/Easter Day dinner. Is this correct ?

yes, it's correct - christmas day or easter day dinner are no big deal

It depends on the family I guess. We do organize such dinners and invite guests/family or let ourselves be invited.

I actually think now that Poles are starting to do a turkey dinner on the 25th- I have no proof of this, just our experience of trying to hunt down a flipping turkey in December. I think that as Poles are starting to return to Poland they are bringing a few traditions back with them.

On Christmas day me and my sister usually ask our mother not to cook dinner.She is always tired after doing all Wigilia (we help her of course but she's doing all the actual cooking) and besides there are so many good things to eat that the dinner would be a waste.Only if we invite someone the dinner is cooked.

It can be that, or a goose, as was the case this past X-mas. And no, somehow it was not an imported tradition. There was simply no one to import anything in my family.

I would say the struggle to hunt down a turkey in December is more down to most shops not stocking a huge amount of whole turkeys. I rarely see whole turkeys in most shops, and even then there isn't a huge amount of them.

I would doubt many families are doing the turkey thing on the 25th, there is such a big meal on the 24th and probably plenty of leftovers so why would you bother cooking another huge big meal, seems a bit wasteful.