There is usually a sale on clothes and accessories on Cyber Monday. They also sometimes offer a small percentage off coupon or free shipping in the catalogues. I've seen them on Jill's Steals and Deals (part of the Today show), but there won't be another of those deals until next year.
American Girl does not do any major sales other than a Cyber Monday which is basically a clear out of old stock. They occasionally have daily deals on specific merchandise or seasonal sales on season specific items. Dolls are never included.
The annual American Girl sale was in July. It's at their headquarters in person near Madison.
The online sales historically have only included outfits and accessory-type stuff. There have been a few Jill's Steals and Deals that included dolls but they are not announced in advance and sell out in literally a matter of minutes. The facebook page for Jill's Steals and Deals says no more American Girl sales this year, so don't look at that as a source of Christmas presents.
They've also been on Zulily a few times this year. Those haven't sold out nearly as fast but the prices are higher than Jill. I haven't heard at all if that will happen again before the holidays but again I wouldn't bank on it since it's completely unpredictable.
The Costco gift cards mentioned above would be the best way to get a deal on a doll at this point.
Jill's Steals and Deals had an AG doll sale in September. It has not been my experience that the dolls sell out in minutes; it's more like hours. There are sales of AG accessories on their web site all the time.
There is usually a sale on clothes and accessories on Cyber Monday. They also sometimes offer a small percentage off coupon or free shipping in the catalogues. I've seen them on Jill's Steals and Deals (part of the Today show), but there won't be another of those deals until next year.
How do you know there won't be another? Typically they do one in Nov or Dec each year. I bought what I wanted this summer, but I still often try for the deal when I see them in case I have a family member who wants one.