Don't worry, now we explain to you about thing it deals with.
So let's start from the translation into Italian from English: throwbackthursday is given by the union of English words throw back (throwback) meaning return to the past e thursday what does it mean Thursday.
So it's about this:
Thursday stars, VIPs, celebrities but also ordinary people who have the desire to join this nice initiative, post old pictures of himself but or of lived situations, in short, memories of the past, and they only publish them on Thursday.
Thursday it's the day when memories are posted, funny, nice, melancholy, fragments of real life to get known and for know better the others.