If you have a costco membership you can get a big bag of seeds there for a decent price, I think they have fertilizer too.  Given Calgary's lack of stable climate, I've never had an issue fertilizing early or late.  I tend to overseed and fertilize and the seeds always sprout well. 
 
If you find a secret for the weeds, let me know! 
 
As for any veggie stuff, its always a gamble here.  May long weekend is usually considered fairly safe, but around her,e it can still snow or frost after.  Last year I planted my potatoes in April and the tops got frosted out once or twice, but they did fantastic.
		 
		
		
		
		
		
		
		
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