Use it any time it's ready.

Im in zone 4 canada and I need to be careful about giving any sort of amendment/fert. too late. I dont want late vigerous growth going into september or october

Compost won't cause vigorous growth in the way fertiliser might.

The affect of the compost would be about the same as far as providing later season N whether you applied it now or had done so in spring. It is a gradual release that peaks in warm conditions if soil is moist and aerated.

Perfect! Thanks everyone