Harvesting grass seed

Was wondering if anyone knew anything about when the beat time to harvest grass seed is, I don't see the point in buying in loads of seed when there's loads of seed around just wanted to know when's the best time and how to tell, cheers, joe

What type of grasses are we talking about? Species? Common names?

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I found information about grasses for eating and grasses for lawns.
Eating:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex134
Lawns:
http://homeguides.sfgate.com/harvest-grass-seed-63971.html