Best time to sow is around April & onward to Sept. But you will probably get growth if soil temp is ok from FEB or March. I'd leave it til March if u can hold off.
Sand spreading is good yearly- use only garden sand. First rake out moss, (particularly in shaded areas if you have any), and only spread a thin layer of sand which can be lightly raked in. You can also prong soil lightly 1st before spreading sand, and brush it in. More time consuming & difficult, but worth the effort. Grass grows roughly in the 1st 5-10 cm of soil. this needs to be free from compaction, but not too loose. Cover seed lightly with sand to avoid bird damage.