Sometimes I get confused about this in English too. If someone says "nächstes Wochenende," does it normally mean the upcoming weekend, or the one coming after that?
Confusing as it may sound, this differs between regions or even between families within a region.
For example, I (and members of my family) would say:
Dieses Wochenende waren wir im Zoo, nächstes Wochenende gehen wir ins Kino, und übernächstes Wochenende fahren wir ans Meer.
(dieses — heute — nächstes — übernächstes)
So, for me nächstes Wochenende is usually the upcoming weekend. By contrast, my friend (and members of his family) would say:
Voriges Wochenende waren wir im Zoo, dieses Wochenende gehen wir ins Kino, und nächstes Wochenende fahren wir ans Meer.
(voriges — heute — dieses — nächstes)
So, for him nächstes Wochenende is the weekend after the upcoming one.
We have very often argued about this misunderstanding, and meanwhile, I usually say kommendes Wochenende to make clear that I refer to the upcoming weekend. But still we often get confused when someone talkes of dies Wochenende.
That's a tough question because even Germans are split about this. As a rough guide, the upcoming weekend might be called "nächstes Wochenende" if the past weekend is just over (e.g. on Mondays and Tuesdays) and "dieses Wochenende" if the upcoming one is just around the corner (e.g. on Thursdays and Fridays). In that case "nächstes Wochenende" refers to the weekend after the upcoming one.
Confusing? Germans confuse each other all the time. So it's a good solution to add the date after "dieses/nächstes Wochenende" (e.g. "dieses Wochenende am 19./20.").
For me, the meaning of "Dieses Wochenende" depends on the context (on the current day of the week): it is the last weekend on Mo+Tu and the upcoming one on Th+Fr.
But however you understand "dieses Wochenende" yourself, you can most of the time differentiate these meanings, as you will use "Perfekt/Präteritum" for the last one and "Präsens/Futur" for the upcoming one:
Dieses Wochenende war ich... -> the last one
Dieses Wochenende gehe ich... -> the upcoming one
"Nächstes Wochende (WE)" on the other hand, will for me always be the upcoming one, which of course means that "diese WE" and "nächstes WE" are the same when around Th+Fr.