Spring arrives in the polytunnel four to six weeks earlier than it does outside, and so spring is when the benefits of tunnel growing will be most obvious to you. Outside things are struggling to get going, wind-swept and frost-burnt, but inside the tunnel it already feels like summer. As soon as the days lengthen noticeably, the tunnel will be up and running even though conditions outside might leave a lot to be desired: so make a start with early sowings and enjoy harvesting your autumn-sown plants, while your neighbours are still waiting for their soil to warm up.
Ventilation can increase as the days grow longer, but nights can still be very cold so make sure that the doors are closed at least an hour before sunset and are opened again at first light. Fleece (floating row cover) protection for tender plants can be removed, but replace it if heavy frost is forecast and take it off the next morning.