As it is pot grown, you can easily leave the plant in its pot for a few days/weeks before planting it. Just ensure you keep it well-watered, and it gets enough sun. Don't leave the plant for too long, however, as you don't want the roots becoming root bound.̴Ì?
In terms of trimming the Leylandii, ensure you trim around Springtime (when the weather begins to warm up). It can also be done in Summertime; however, you must make sure there have been no dry spells or very hot weather. As well as this, it‰ۡó»s also important to notice that Leylandii does not grow from old wood, so therefore cannot be left to go way out of hand and then suddenly trimmed down to size.