There are many types of Lavender, but the two most common that we grow in our gardens are varieties of English lavender (Lavandula angustifolia) which tends to flower from June onwards and varieties of French Lavender (Lavandula Stoechas) which tends to start flowering from May.
Lavenders are prone to getting woody if left unpruned, so it's important to trim them back each year to keep them vigorous and in a good shape. Timing is the most important thing with pruning lavenders and it needs to be completed by the end of August to ensure that the wounds heal over in time for winter. This can be hard as English Lavender may still be flowering, but it's important not to do it any later. They may look a little sad after pruning, but the timing of it will allow them to start re-shooting with a little mound of tougher leaves to see the plant through the winter.
The other most important thing is to make sure you don't cut into the old wood, i.e always ensure there is an inch or two of green shoots beneath where you are cutting. Generally, you'll find that you're cutting around half of the greenery off each time (unless they are young plants). If you were too late to cut them back by the end of August or you think you want to cut them back a bit more to shape them, they can have a light prune in mid -late spring also, but it will delay the flowering time.
If you have French lavenders, it pays to keep deadheading the flowers once they have finished and they will give you a second flush of flowers later in the summer.