I would think before Christmas because the companies would want people to actually buy the phone. Then again, possibly after, as an incentive to buy since after Christmas is usually a slow time for retailers. It would more than likely depend on the place of business or the cell phone manufacturers.
The most common time for Christmas discounts would be on what's called Black Friday. This is the Friday after Thanksgiving where very nice deals are given, but deals run the whole season and past Christmas.
Price Network is a Canadian website that offers deals and discounts to it's members. Typical deals and savings include money off cameras, cheaper groceries and comparisons of mobile phone contracts.
There are often good deals on eBay. Also, when you buy your phone through amazon it is many times cheaper than how much it retails at your cell phone service provider.
Both T-Mobile and Verizon offer great deals on their family plans. T-Mobile is slightly cheaper, but other than that they both offer similar features, so it really depend on what's best for you.
On Christmas Eve
They are usually better after, when most prices go down and things go on sale. Though Black friday seems to be the best day for sales which is before christmas.
Oftentimes Christmas decorations will be on sale before the "main" Christmas season so you may find good deals at Halloween time. Usually peak time is when they will be most expensive.
Information on T mobile phone deals is available on the T mobile website, as well as retailers of mobile phones, such as Best Buy, and T mobile kiosks.
Many all stores will have deals available for the items that were not sold out after Christmas.
Local electronic retailers usually have deals for mobile phones. Online, one can use sites like dealnews, where deals for mobile phones are given articles on a regular basis.
Mobile deals on the 3 network can be found in a few places. These places include BuyMobilePhones, E2Save, AffordableMobiles, and DealsOn3.Mobile deals can also be found on 3's official site.
I always find it is best to get computer from Best Buy, but check Pricegrabber.com. It may be cheaper to get it right from the Dell site. Look for deals around Christmas time.