I am concerned about the summit day. From the books and reports I realize that it could easily be up to 16 hours day (11 hours ascent, some time on summit, and over 4 hours descent) and temperature around -20. What is the way to keep water not frozen in such conditions? Any suggestions would be appreciated.

Good mountaineering jackets have pockets on the inside for water bottles. If you have the bottle close to your body it wont freeze.
A thermos is another solution.
Some mix their own "summit day liquid", with ingredients which can take a lot of cold before it freeze.
A very special type of this is the Russian vodka-honey-milk mix. Perhaps not recommended

On the evening prior to summit day last January, I slept with three, full, 1 liter Nalgene bottles in my sleeping bag at Camp Berlin (along with my urine bottle!). I had melted the snow just after dinner. I used insulated bottle covers (2 from OR, 1 from a generic company) on the trip up tthe next day without any problem. Kept the insulated bottles in my summit pack.

Hope that helps

I was on Aconcagua both last year and just returned from climbing it this year and never saw any temperatures that cold. This year we boiled 2L of water each the night before summit day and put them in their insulated parkas inside our sleeping bags. On summit day the water was fine.

Bring a thermos as well. It's a nice way to have hot liquids anytime, including summit day.