There are a few things that affect the outcome of a photograph when photographing outside:
• Time of Day
• Time of Year
Time of Day
There are 7 time periods during a day when there is light to make photographs outside
Dawn -- this is the 20 to 30 minutes before sunrise. The light at this time of day has a blue color-cast to them. Also, there are really no shadows to contend with in the photos.
Sunrise -- There are two "Golden Hours" in the day. The Golden Hour the first and last hour of sunlight during the day. The sun's angle will produce long shadows at this time of day. Where the sunlight illuminates appears as gold in color. The contrast is less during the golden hour, shadows are less dark, and highlights are less likely to be overexposed. The dynamic range is less than midday and therefore you can record more of the scene than other parts of the day.
Morning -- The light can still be pretty good at this time of day. The sun is up and can still shine on a person's face so that you can still see their eyes and the color is more neutral.
Midday -- This is when the sun is directly overhead. The dynamics range is great and therefore your photos are more contrasty. You either expose for the highlights and the shadows are black or you expose for the shadows and the highlights are blown out. It is very difficult to find a middle exposure to hold it all together. Since the light is directly overhead more often people's eyes are in the shadows and you often loose the detail of the eyes. You need fill flash to show people's faces at this time of day.
Afternoon -- Very similar light to the morning, but a little warmer due to the atmosphere being warmed up through the day.
Sunset -- You need to know the time of sunset to maximize the Golden Hour before the sun disappears below the horizon. The reason you see a lot of silhouettes of people and sunsets verses sunrises has more to do with people being awake.
Dusk -- I like to shoot architecture at this time of day, because you can put your camera on a tripod
Time of Year
There really isn't a "best" time of year to photograph. However, my favorite time of year is the fall. I love the cool weather and all the color in the leaves. I also like the fall since there are more high pressure days, which translates into less stuff in the air making it where you can see further on clear days.
No matter the time, as long as you have light you can make photographs. Sometimes shooting at the time of day many avoid will make your photos different. My suggestion is to always try and shoot the Golden Hour when you can outside and then look for other opportune moments to capture people and places in the best light for them.