Preaching the Gospel, Preparing a People
Felix, you admit that you don't know ("might not be"). I feel fully convinced that it does matter and that we can rest assured that we are observing the correct days of worship.
Let's approach this from a different point of view.
Imagine that you're God and you've created mankind. In your creation you give a very strict command to observe specific days and feasts, all at very specific dates and times (sunset). As God you know man will ignore and change everything that you have set in motion ("Daylight Savings Time" a perfect example). Yet as God you are concerned with the people that you are working with and you know that it's your responsibility to make sure that they have the ability to worship you according to your original command. We know by scripture that God is the same today and throughout eternity (Hebrews 13:8). So what He expects from us never changes just because man tries to mess with what the Almighty has set into motion. He requires the same obedience from us as He did from Adam and Eve. Therefore He knows that He must give His people the right calendar and days to properly worship Him. God will always provide for His people. Gen. 22:14