Usually people say "How was your weekend?" But weekends are two days right? Then why don't we say "How were your weekends?" (If we are asking how his/her weekend was then which one of the two are we referring to :) )
One weekend is made up of two days. I ask about the singular item unless I have a specific reason for the plural "weekends."
The singular "weekend" is more common.
I would say "How was your weekend?" (singular) as equivalent to "How were your days away?" (plural).
If I had not seen my friend for fifteen days (a period of time which covers two weekends), only then would I say "How were your weekends?" and I would say that only if I wanted to know how four of the fifteen days were for my friend.
This is similar to how a week is made up of seven days. I would say "How was last week?" when referring to that one single block of time. If it was four weeks since I last saw my friend, I would say "How were the last few weeks?"
Any object that is a collection is itself singular.
How was your weekend (a collection of days that is known to most parties)?
How is your soufflés (a collection of ingredients)?
There are some collections that contemporarily pluralized... pants is a good example.
How do you like those pants? How do you like that pair of pants?
Both can be asked of the same garment, but it's an oddity.
If the question is in opposition to "how were your weekdays?" then the correct question would be "how was your week (a collection of days)?"