In both your examples it's impossible to be sure, without further knowledge of the context, whether "a" or "the" is more appropriate. Generally speaking, if the specific thing that you're talking about is already known to the listener/reader then you use "the". Otherwise you use "a".
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AwenceThere is no preceding nor ensuing sentence after it. Hence i have no idea, whether the procession is a known event or not.Sometimes common sense helps. This is your sentence:
AwenceThe question is, the sentence i mentioned above appears in an English exercise. There is no preceding nor ensuing sentence after it. Hence i have no idea, whether the procession is a known event or not.