每个节日商家都会打折扣。没有节日制造节日也要打折扣。

纯粹就是为了打折卖东西，其实也没有便宜多少，我室友们都在买，结果双十一那天服务器都挤爆了

11.11，象征着单身，光棍，‘1’看起来就像是一个光溜溜的棍子一样，就是单身的意思。至于折扣，这不过是一种促销手段，在节假日的时候，大家都喜欢过节的气氛，所以在这种时候，快快乐乐的一起买东西是一个不错的选择，因此才有了这种默认的约定。

As @Huimir said.

一/1/one is an odd number, meaning single, only; while 二/两/2/two is an even number, meaning couple, double. Normally, even numbers are fortunate numbers in China. And there is proverb called 'Good things come in pairs/好事成双'.

So 11.11 is so singggggggggle, unununununfortunate,...

'Singles' Day' literally means "bare sticks holiday". It is a day for people who are single, celebrated on November 11 (11/11).

First, it was a ceremony, day meaning you are not single any more/脱光节, so many people choose today, 11.11, to get their marriage licenses, to make their confession to their beloved.

Afterwards some online e-commerence companies like Alibaba Group, JingDong, etc. creatively invented this carnival(双11/双十一/double eleven) for shoppers and shopkepers ,akin to 'Black Friday (shopping)' in America. Some people say in a kidding way that this carvinal is a opportunity especially for boys/men to show how much you love your girls. "Girl:'Love me?' 'Go shopping for me." So it can also be related to 'Singles' Day'/'Guanggun Jie'/'Tuoguang Jie'.

1 http://baike.baidu.com/view/23105.htm#1_1

2 http://baike.baidu.com/view/6699158.htm

3 https://en.wikipedia.org/wiki/Singles_Day

4 https://en.wikipedia.org/wiki/Black_Friday_(shopping)