For guests coming from abroad, the most convenient time to fly to Poland would be Wednesday evening/night. Thursday morning is also a possibility, but it may be tight if there are flight delays or accidents on the road, as the ceremony starts at 3PM sharp.
Should anyone arrive later than this, we would ask them to wait until the ceremony is over at 4PM and meet us outside the church, as to not disturb the Holy Mass, or head straight to the wedding venue.
The nearest airports are in Gdańsk or Bydgoszcz. Gdańsk is a much bigger airport with more frequent flights - most leave from London Stansted - so that would be our recommendation. It's also much easier to spell.
Bydgoszcz airport is much smaller and offers fewer options to get pissed before your flight. You would also need to suffer the indignity of being in Bydgoszcz.
The best way to get to Grudziądz from either town is by car, which should be easy to hire from a company near the airport.
The venue, Rytebłota Resort and Spa, is approx. 1 hour 10 mins from the church. The drive is quite scenic but do not trust Polish roads or drivers.
This is the route we recommend you take, as the alternative way goes through even more villages: