If I was to make decorated sugar cookies tomorrow (white royal icing) and was to put them in an airtight container and freeze, would they taste fresh on sunday?

better question....if i decorate tomorrow and just store in an airtight container at room temp, will they be fresh on sunday?

Freezing decorated cookies may lead to color bleeds and cookies not looking so nice. I have a cookie decorating book and I checked on this.   Most cookie recipes stay fresh in airtight containers up to 10 days.  I don't have a problem keeping mine but everyone is different with their cookie expectations.  My recipe is a soft cookie while most others are a crisp cookie.  Crisp cookies tend to have a fresher taste longer; if that is your recipe, I wouldn't freeze them just keep them in an air-tight container.  I made some for Easter but didn't decorate them so mine are in the freezer until Saturday night. 

Hope this helps.

If you freeze them they will definitely taste fresh, but that wouldn't be my main concern.  The issue is how the icing will survive the freezing and thawing.  Cookies kept in airtight containers will be fine on Sunday.  I have had mixed results with freezing royal.  If the icing is not completely dry you will get splotchiness.  If you do not thaw correctly you can get bleeding.  I have decided there is too much work involved to risk freezing unless the order is just too big to do in a reasonable amount of time.

so if i decorate and store in airtight container they should be fine out at room temperature on sunday? that would be wonderful and save so much time!

however egg whites are ok at room temp that long?

With the addition of all the sugar they are no longer egg whites but more like candy. They'll be fine.

As an alternative just use meringue powder.