Your lawn does not completely become dormant in the winter. You will note that even when the temperature outside is below freezing or just a few degrees above, the top of the leaf blade will become fluffy and produce a new spike fairly soon after the last cut.
It is important not to let the grass get too long because the stress of removing a great deal of growth in one swoop can cause damage with moss and weeds infesting the surface that is vacated by grass that is dying away.
A little and often approach is the wisest one and in that respect, there is not really clearly defined time for the first time in the season you should mow the lawn.
First and foremost, make sure that your mower is ready to go. Servicing and blade sharpening should be done well in advance and at the first signs that your lawn is adding a little growth you should consider a cut.
Never remove more than a third of the leaf blade in just one cut. If the grass has become a little too long, raise the cutting height, cut the lawn and wait until the leaf blades stand erect again and after a couple more days, give it another trim.
Try to avoid cutting when the surface is either very wet or frozen.