Autumn to spring is the ideal planting time for most trees and shrubs as they are not actively growing and there will be less likelihood of stress to the plant. However, it is best to avoid planting in waterlogged or frozen ground.
If the soil is waterlogged do wait until surface water has drained away before you plant. If the soil consistently remains wet consider improving the drainage before planting or the plants could suffer in the long term. Walking or driving on, or digging the soil when waterlogged can also damage the structure of the soil - compacting it and reducing the aeration which is necessary for plant roots to thrive.
Generally, if there is snow on the ground or the ground is frozen for several days it is advisable not to plant. Do not remove pots or containers from the root ball during freezing conditions as the small roots can be damaged.
Bundles of bare-root plants can be kept in the bags they are supplied in, in a shed or garage for about a week prior to planting, if the delay is longer they are best 'heeled' in the soil in their bundles or in a free draining container with compost around the roots. Rootballed plants are best left out of drying, cold winds with straw or hessian over the rootballs. Container grown trees and shrubs are fine in the containers they come in until planting conditions are suitable - just secure them safely where they won't blow over.