When taking creatine definetly use it on workout days around an hour before lifting and within a short period after your finished. It has been argued on this forum several times if you should take it on off days(a lot of people say no). If your taking creatine you might as well go through with the whole cycle instead of half assing it, that way you'll know if this supplement truly helped you out with gains etc. Hope this helped a little bit.
If you missed a day, don't fret. Creatine is something your liver produces daily anyway, by combining 3 amino acids, Arginine, Glycine and Methionine. It is not magic, just pick it up the next day.
I used to only take it on workout days which worked out to be 4 times a week, but now I take it on cardio days as well (so 7 days a week) and I noticed a difference. I think I am sold on taking it every day although it doesn't ruin you to miss a day.