To illustrate the point in more detail: let's assume you are supposed to have dinner with your mother-in-law on Sunday; what would you tell your friend?
You may choose between : "W niedzielę zjem obiad z teściową" and "W niedzielę będę jadł obiad z teściową".
Both would be perfectly acceptable, so you may simply choose the former one. But the majority native speakers will subconsciously want to choose the latter while wishing to reflect the duration of the event. In doing so, they will say that they will be meeting and talking with others during this most supposedly family meeting to which their dearest or not-so-dearest mother-in-law will be come along. As this is perhaps an important or unusual event, people might - subconsciously again - say: "Mama przychodzi dzisiaj do nas na obiad" in which sentence all her and our preparation for the event that clearly takes a certain amount of time are rendered by using the imperfective aspect - the duration really MATTERS here! To say: "Mama przyjdzie dziś na obiad" will make it look quite common, and indeed would be fully justified when shown in perfective aspect, if such an event happens fairly often and we are rather used to it.
In reporting a similar even in the media, people will typically use the perfective aspect, saying, for example, "Kanclerz Agela Merkel zje dzisiaj obiad z premierem Donaldem Tuskiem", as it is quite irrelevant to underline the duration of the event here; the headline simply informs that Bundeskanzlerin Merkel "spotka się dzisiaj z premierem Tuskiem przy obiedzie".
You may choose between : "W niedzielę zjem obiad z teściową" and "W niedzielę będę jadł obiad z teściową".
Both would be perfectly acceptable, so you may simply choose the former one. But the majority native speakers will subconsciously want to choose the latter while wishing to reflect the duration of the event. In doing so, they will say that they will be meeting and talking with others during this most supposedly family meeting to which their dearest or not-so-dearest mother-in-law will be come along. As this is perhaps an important or unusual event, people might - subconsciously again - say: "Mama przychodzi dzisiaj do nas na obiad" in which sentence all her and our preparation for the event that clearly takes a certain amount of time are rendered by using the imperfective aspect - the duration really MATTERS here! To say: "Mama przyjdzie dziś na obiad" will make it look quite common, and indeed would be fully justified when shown in perfective aspect, if such an event happens fairly often and we are rather used to it.
In reporting a similar even in the media, people will typically use the perfective aspect, saying, for example, "Kanclerz Agela Merkel zje dzisiaj obiad z premierem Donaldem Tuskiem", as it is quite irrelevant to underline the duration of the event here; the headline simply informs that Bundeskanzlerin Merkel "spotka się dzisiaj z premierem Tuskiem przy obiedzie".