Autumn is best because you then have all winter for the dressing to be taken down into the soil. If applied now and we get a warm dry spell then it may just dry up and sit on top of the grass and then be mown off again next time you need to cut the lawn.
If you are just wanting to re-vitalise your lawn then try using a product called After-Cut by Westland Horticulture. It is very good and easy to use.