The best time to stretch is when your muscles are warmed up. If they are not already warm before you wish to stretch, then you need to warm them up yourself, usually by performing some type of brief aerobic activity (see section General Warm-Up). Obviously, stretching is an important part of warming-up before (see section Warming Up), and cooling-down after a workout (see section Cooling Down). If the weather is very cold, or if you are feeling very stiff, then you need to take extra care to warm-up before you stretch in order to reduce the risk of injuring yourself.
Many of us have our own internal body-clock, or circadian rhythm as, it is more formally called: Some of us are "early morning people" while others consider themselves to be "late-nighters". Being aware of your circadian rhythm should help you decide when it is best for you to stretch (or perform any other type of activity). Gummerson says that most people are more flexible in the afternoon than in the morning, peaking from about 2:30pm-4pm. Also, according to HFLTA, evidence seems to suggest that, during any given day, strength and flexibility are at their peak in the late afternoon or early evening. If this is correct then it would seem to indicate that, all else being equal, you may be better off performing your workout right after work rather than before work.
On the other hand, according to Kurz, "if you need [or want] to perform movements requiring considerable flexibility with [little or] no warm-up, you ought to make early morning stretching a part of your routine." In order to do this properly, you need to first perform a general warm-up (see section General Warm-Up). You should then begin your early morning stretching by first performing some static stretches, followed by some light dynamic stretches. Basically, your early morning stretching regimen should be almost identical to a complete warm-up (see section Warming Up). The only difference is that you may wish to omit any sport-specific activity (see section Sport-Specific Activity), although it may be beneficial to perform it if you have time.