Is GPS more accurate on specific hours of the day?

There are a lot of factors influencing GPS accuracy. James Ryan is right about night time and clear atmospheric conditions. Then there is the satellite "layout" on a specific time at at specific position on earth. Depending on how many you can "see" (4 at least for a 3D fix) and their distribution (all in one row is bad, evenly spread out better) your accuracy will change. An index for accuracy is the Dilution of Precision which can be calculated from the satellelite positions. The best value this index can get is 1 (pretty much only in theory) but usually is somewhere around 2-4 when everything is normal. Times when the DOP is greater 5 should not be used for field recordings (if you want good results).

DOP can be calculated ahead of time using recent ephemeris data. This way field campaigns can be planned for maximal accuracy. Trimble offers an free planning software for that.

Great Question: Can Time and Position affect your GPS accuracy.

Short Answer: YES

Why?

First lets place your GPS Receiver in perfect conditions where the atmospheric conditions are perfect, there are no multipath effects, no radio interference, and line of sight to GPS Satellites is clear.

Your Time and Position on Earth will determine where the GPS satellites will be in relation to you. Relative to your earth postion, the GPS satellites have their own positions that will give you a maximum accuracy and precision GPS Location. Any deviation from this theoretically perfect alignment between you and the GPS satellites will degrade accuracy to a point where no accurate position can be computed.

Besides the Windows Tools provided by other very helpfulls, I really love the NAVCOM online website tool for quickly checking GPS satellite availability and predictive DOP in TIME relative to co-ordinates given.

NAVCOM GPS Satellite Predictor Tool

Great tool if you require a certain level of (x)DOP for GPS Data Collection, to prevent spending an entire day in the field due to poor (x)DOP of GPS Satellites. Of course this tool can not calculate other external factors which will affect (x)DOP.

Accuracy is better at night and in clear weather as the state of the atmosphere is more consistant and predictable. This is particularly the case if you are using WAAS or EGNOS to improve accuracy.