Lilacs can be planted in spring once the ground has thawed or in the fall  before the ground freezes. In spring, lilacs are shipped dormant and are likely to have bare roots and no leaves or buds. Please be assured, the lilac is not dead, only "sleeping." Upon arrival, remove any packaging from the root system and soak the roots in tepid water for 10 to 15 minutes. 
In late summer or fall, lilacs may be shipped with soil and have a complete
root system with leaves on the stems. Upon arrival, soak the roots and root ball in water for 10 to 15 minutes. 

How to care for the lilac if you can't plant right away: 

If you can't plant the lilac right away, soak the roots as described above, then plant the lilac temporarily in a holding bed. Set the lilac at an angle ("heeled") and entirely  pack the roots with soil. Add additional soil and keep the soil moist until you are ready to plant. 

Lilac care

There are four basic rules to follow in taking care of your lilac(s): plenty of sunlight, good drainage, provide good soil and pruning.

Choosing the planting site

Avoid planting lilacs along walls or among large trees (or trees that will grow tall). Use complementary shrubs, plants, or other garden outcroppings to enhance the appearance before and after bloom. Space lilacs no less than 6 to 10 feet apart. Crowding requires more frequent and drastic pruning. 

Sunlight

Make good use of available sunlight; try a south or southwest spot out of the way of doors or windows. Lilacs require a minimum of six hours of direct sunlight daily. The amount of sunlight dictates the appearance, color, and quantity of bloom. Too much sunlight is better than not enough. 

Drainage

Good drainage is characterized by the soil's ability to retain sufficient
moisture to nourish the root system while still being able to drain off excess moisture. Lilacs do not thrive in soggy soil. 

Before planting, try digging a hole about 8 inches in diameter by 12 inches
deep. Fill the hole with water. If the water has not drained after one hour,
improve the drainage or move the plant to another site 

To improve drainage: 

1.Remove the topsoil from the actual planting site (an area equal to 2
to 3 times the lilac's root system) and reserve. 

2.Mix sand and/or fine gravel 6 to 10 inches deep into the subsoil (not
the topsoil) 

3.Mix the reserved topsoil with peat, vermiculite or other porous amendment to cover the root system when the lilac is planted. 

Planting 

1.The planting hole should be deep and wide enough to accommodate
 the plant's root system. We recommend adding compost, bone meal or an all-purpose fertilizer to the planting hole. If your soil is acidic,
adds some ground limestone. 

2.When planting, place the top of the root ball level with the surface of the hole. If the lilac is bareroot, the top layer of roots should be a few
inches below the surface. When filling in with soil, it is important to
water well, but do not flood, and avoid compacting the soil around the root system. The idea is to remove air pockets, yet keep the soil
porous. 

Remember to water your lilacs regularly throughout the summer.
During the dry season, water more frequently to keep the leaves robust, not limp. 

Fertilizing

Fertilizer should be applied at the base of the plant early each spring to help provide the plant with nutrients for the coming year. Buds are set the
previous year so the fertilizer will feed this year's leaves and next year's
bloom. We recommend our Organic Flower Fertilizer. Lilacs love a sweet
soil. If your soil is acidic, adding horticultural lime in the fall will help the soil stay alkaline. 

Mulching

Using mulch will help hold water in the soil and reduce heat stress. If you
see the leaves getting limp during summer it is a sign that the plant needs to be watered. 

Deadheading

To encourage repeating blooms, the Josee Lilac must have its dead flowers removed after each bloom. When blooms have faded, remove the flower heads by cutting just below the individual flower heads. This will stimulatethe production of new flower and leaf buds on the plant. All lilac varieties will benefit from regular deadheading. 

Pruning

Lilacs do not require annual pruning, however cutting off blooms from main
stems within a week after blooms have faded will help the plant concentrate on preparing more flower buds and not seeds. If your lilacs become too tall, and the number of blooms declines, you can rejuvenate the plant by cutting 1/3 of all main stems that have a diameter of at least 1-1/2 inches. 

Cut these main stems down to 12 to 15 inches from the soil. This will
stimulate the growth of new shoots. Pruning in this way over a three-year period will refresh the plant while it still continues to flower. 

Over wintering

Lilacs are very hardy and don't require any special precautions for winter. 

Article Courtesy of Gardener's Supply Company