Will the day/night cycle change when the poles melt? I want to know what will happen to day and night if poles melt i think it will change but I don't know why? ^_^ 
 A: To a first-order effect, there would be no change.
But one consequence of melting is that the water moves to other places.  Water that moves from the poles to other areas on the surface of the earth would serve to (slightly) increase the moment of inertia of the planet.  This is because the mass of the water would be farther from the rotational axis.  The result of this would be a slight slowing of the earth's rotation compared to the situation where the ice remains intact.
A: First, two remarks:


*

*The melting of the ice in the Arctic ocean would have zero effect. What matters is the ice over Greenland and Antarctica.

*There is a very long-term secular slowing of the Earth's rotation rate due to the recession of the Moon from the Earth. This answer ignores that effect. (Alternatively, this answer has this secular effect subtracted out.)



Summary
The Earth would rotate slightly (very slightly) faster after all effects of the melting have taken place. Note that this is diametrically opposed to BowlOfRed's answer. I'll look at two scenarios, one in which the ice melts extremely slowly, over the course of several tens of thousands of years (or even more), and another in which the ice melts in just a few thousand years (or even less).
Scenario 1: Melting is very slow
In this scenario, the melting is so slow that the Earth will more or less be in isostatic equilibrium at any point in time during the melt. Greenland has been covered with ice for at several hundred thousand years, Antarctica for several millions of years. Greenland and Antarctica are more or less in a state of isostatic equilibrium, and with a very slow melt, they will remain in isostasy. The melting of the ice transfers water mass from polar regions to the equator, but the glacial isostatic adjustment (aka post-glacial rebound) that results from this melting transfers rock mass from the equatorial regions to the poles. The latter dominates over the former. The end result: The Earth's rotation rate speeds by a tiny bit.
Scenario 2: Melting is very quick
In this scenario, the melting is so quick that the Earth is temporarily out of isostatic equilibrium shortly after the melt occurs. This is a shock, a cause that is much shorter than the system's time response. The immediate response is that the transfer of water mass from the poles toward the equator makes the Earth's rotation rate initially slow down a tiny bit. The Earth however will slowly react to that shock and transfer rock toward the polar regions. This will eventually increase the Earth's rotation rate.
Analysis
The end results of the two scenarios are identical. Tens of thousands of years after the polar icecaps are gone, the Earth will be rotating slightly faster than it was when the poles were covered with ice.
The Earth recently escaped a glaciation period. Vast ice sheets covered much of North America, northern Europe, and Asia 20000 years ago. This ice melted rather quickly around 12000 years ago. That quick melt closely matches my second scenario. Areas of northern Canada and northern Europe are seeing declining sea levels (more land is getting exposed every year). This is because of glacial isostatic adjustment of the underlying land.
People have been keeping track of the length of day since the early 1600s. Two long-term trends are obvious from ~1600 to ~1997: The secular decrease in the Earth's rotation rate due to the recession of the Moon, and a cyclical (but apparently secular) increase in the Earth's rotation rate due to the ongoing glacial isostatic adjustment from the last glaciation. (Something happened in 1997, but that's a different question.)
