I want to get a better mobility and flexibility. A first goal I have is touching my feet with outstretched legs. Also, I want to be able to lift the outstretched leg into a horizontal position while standing upright. How can I accomplish that? Is it effective to do a stretching workout on one day a week? Or is it better to stretch before a 'normal' workout? I really appreciate any help!
There are a lot of activities you can do. You can do activities where the flexibility is a by-product, and important to the activity, but not the main goal, itself - dance, yoga, martial arts are a few obvious candidates.
If you want to improve your flexibility, at all, stretching once a week won't cut it. If you want to drastically improve it, daily is more like it. Maybe you could see some decent improvements with every other day, but much less than that probably won't do much of anything for you.
If you are exercising with other activities more active or "ballistic," movements as part of the warmup are okay. Traditional static stretching before a workout is pretty useless and actually increases the likelihood of injury. That kind of stretching is much more beneficial as part of your post-activity warm-down.
My daughter is flexible to the contortionist level. She says she spends about 45 minutes every evening exclusively doing stretching/flexibility work.