To give plants the best chance of success, make sure you are planting bulbs or tubers in the right depth of soil, even if they are in pots rather than the ground.
Shallow seed trays are not right for large bulbs like Lilium but you can use them to start Begonias into growth.
Fill your containers with good quality soil and plant your bulbs to give your summer selection a head start.
Keep them in a warm frost free place such as a greenhouse to make sure the bulbs and tubers are protected against the vagaries of the spring weather to give them change to establish this will also protect the young shoots from the ever hungry slugs that prowl the garden.
When the weather warms up and summer has definitely arrived, you can move your plants into your beds and borders.
Just be sure before you place them outside that there is no chance of late frost which can take you by surprise.