The early bird catches the worm
Lawn watching in December can save you work in the summer. When it’s very wet or after snow melts, keep an eye on any areas of the lawn which don’t drain as quickly as the rest. These are places that will need treatment, probably scarifying and aerating at the earliest opportunity.
They are likely areas harbouring thatch and moss, preventing air and water getting to the roots of the grass. You won’t be able to do anything much just yet, except perhaps some raking of thatch on drier days, roughing the area so you have a marker on it for later. But come spring you can be the early bird, out there with scarifying and aerating attachments for your garden tractor or using a walk-behind machine.
Keep an eye as well on how shade might be affecting the lawn. Make a note – it might be a tree or large shrub needs some judicial pruning to improve the light on the lawn. Talking of worms, their casts make a bit of a mess but if it’s wet, leave them. Wait until you have a dry, sunny day (they do happen in December) and simply brush them out.