With the bright sunny afternoons it is worth getting young plants outside for a few hours. Young plants crave the sun more than anything so even gentle protected exposure is very good for them.
A cold-frame is an ideal intermediate step for moving plants from the house or greenhouse out into the garden. At this time of year it is still too early to leave the plants outside overnight as temperatures can fall dramatically at night.
Protecting seed trays with plastic while they are outside ensures the young plants maintain warmth and are protected from cold winds. Even on a sunny day a sharp cold wind can wreck havoc with seedlings and kill them easily. It is important to protect young plants from wind.
At this early stage of hardening off the aim is to reach the stage where plants can be left out overnight. This is a tricky business as day time temperatures of 11 degrees can fall considerably during the night and with any wind at all, night time temperatures can fall to 1 degree, killing most seedlings.