A friend of mine told me that the Rabbi who is going to perform the ceremony at his daughter's wedding in December says that there is a time period around sunset when he is prohibited from performing the ceremony. My friend said that he is having trouble getting an answer as to what the problem is. Does anyone know why a marriage cannot be performed around sunset?

I think it may have to do with the Kessuva being predated. There are those that hold that if you make the proper acquisitions during the day, then it's not a problem.

Rav Chaim,
What's wrong with the kettuba being "predated"?
Shalom

This would be called a Shtar Mukdum. Which means it's predated, thus the time is before the actual transaction. Therefore, all property that was sold after the date that is written in the document would have a lien on them to pay off the Kesuva, though there shouldn't be a lien only for those that were sold after the actual transactions. This would cause a lien on properties that there shouldn't be a lien on, thus making the document invalid.

So a kesuva written immediate after sunset is good for 23 hours? but a kesuva written immediately before sunset is not good for even 1 hour?

I would be happy for one of the GY Rabbis to clarify my understanding on this usbject as it related to my wedding too. For my wedding, the date on the ketuba was for the Hebrew day before sunset. If I understand correctly (I am not a Rabbi), the wedding should then ideally be held before sunset i.e. on the same day as written in the ketuba. My wedding was held after sunset but I completed the "kinyan" on the ketuba before sunset, and thus according to some authorities the ketuba is still valid and does not have to be changed. I was told that I do not need to change my ketuba. Is this a correct Orthodox opinion?

Thus, if the wedding is to be held after sunset, then the date on the ketuba should be dated after sunset .
But between sunset and tzeit hakochavim (when the stars come out) it is not clear whether it is halachically night or day (and what the correct date should be on the ketubah) and thus many people do not hold weddings during this time but wait until it is definitely dark (i.e. until after tzeit hakochavim).

That seems accurate.

B"H

Since this thread has brought up the subject of "Ketubos" (Marriage Contracts), there is an excellent URL link that shows online "Ketubos" from the various centres of world Jewry. Just click into the link and into the country desired.
http://jnul.huji.ac.il/dl/ketubbot/html/country_list_graphic.htm

Sincerely,
David