You can feed your lawn anytime but I’d suggest waiting until it is greening up on its own. This insures that the soil is warm enough for the lawn to take advantage of the food. If you want to use a lawn food/crabgrass preventer combo product then you’ll need to wait until the lilacs are just starting to flower to apply that. More on that HERE. Moss likes acid soil so ongoing pH testing an lime applications will help keep moss from getting started but, once it has, lime can’t kill it. More on pH testing HERE. Here’s a link to more on using moss control…something that should be done in the cooler days of late April/early May,