When you say weeping willow, I'm going to assume you mean a standard Kilmarnock Willow rather than the proper Weeping Willow, which is a huge tree usually seen in parks near water. Working on that assumption, you can plant it out now, provided you're careful with the rootball. Would have been best to do it a month or so ago, but never mind - make sure the hole you prepare is plenty large enough and deep enough to accommodate the roots. Technically, you should unwind any coiled roots, but as its in flower currently, I wouldn't. Add some fish blood and bone to the bottom of the hole, and keep well watered during dry spells this year.
As for the primroses, depends where they were when you bought them - they are fully hardy, but if they were inside or under shelter at point of sale, you may need to harden them off for a few days first.