Marriage invitation

We got invited by John,a british gentleman to join the marriage of his sister in law tomorrow at sunday.
Unfortunately we would have to leave early because Danny have to get up at 6.40 am at the next morning for school.So we thought it would be quite impolite to leave only one or two hours after the beginning.
That's why we choosed to reject John's invitation with many apologies.Thank you anyway very much and sorry again,John.:)