Ask Mr. Smarty Plants

You don't mention what type of lilies you purchased so I am assuming they are summer blooming Asiatic hybrids.  We cannot give you any really specific advice about plants unless they are native to North America but can offer some general advice in this case.

You can plant either your lilies or your "cedar"as soon as the ground is "friable"; that means that it is thawed and has dried enough that you cannot squeeze any water out of a handful.

As far as the lilies go, you will want to follow the directions on the package as we don't know exactly what they are. Whether or not you add any "amendments" to the hole is up to you.  Some people add bone meal when they plant bulbs and root stimulating fertilizer when they plant shrubs and trees.  Other people believe that it is better to allow the plant to get established more gradually and just keep it watered (but not saturated).  It is not necessary to make the hole much bigger than the pot but it is very important to loosen the roots when you plant.  That will promote the roots' ability to penetrate the undisturbed soil surrounding the planting hole.  Check out these links to publications by Tree Canada and Canadian Living.

I hope your cedar survived the winter not being in the ground.  The biggest challenge with a Christmas tree in a pot in an environment where you can't plant it after Christmas is that it is too warm and dry inside for it to go into its necessary dormancy and it is too cold outside (even in the garage) for the pot to be left above ground.  It is a good idea to dig a hole before the ground freezes and keep an unfrozen mix of soil, leaves and compost so that the plant can be heeled in for the winter.  By now, though, you are likely aware of whether it survived or not.

One of the advantages of planting native plants is that they are adapted to your natural conditions, rarely need coddling and often provide a wildlife benefit.  There are some lilies that are native to Quebec:

Lilium canadense (Canada lily)

Lilium philadelphicum (Wood lily)

If your "small cedar" is our native Thuja occidentalis (Arborvitae) be aware that it can ultimately become quite large so choose your location with that in mind.  It will respond well to being pruned (there are plenty of cedar hedges in Quebec and Ontario), especially by the deer (and there are plenty of "lollipop" cedars as well)!

Lilium canadense

Lilium philadelphicum

Thuja occidentalis