Shizue san, Konnichiwa!

Thank you for sharing this experience with others!  I would like to suggest a few edits to your post [ … ]:

This ‘Obon’[You would start this sentence with “Obon”, or rather, “The festival of Obon”], as well as being a religious event to commemorate the souls of ancestors, has the air of a national holiday, with many people traveling around the country and, for [you can use a period here: “.  For”] those not aware of Buddhist practices, [the] Obon [is celebrated] around August 15[period “. It ”] is just a summer vacation, though most people throughout the country maintain the tradition of visting [visiting] family graves. this is web site. [“This is the web site” --did you mean to provide a hyper link?]

I [was] grew up at [by] the seaside. The tray [? “tray” is a big dish used to carry glasses at the restaurant] was told that a dead person returned from the waterside. "Okuribi" is one of the events observed in Bon festivals to usher out from this mortal world to the other world those spirits of the dead that had once returned to their old homes during the Bon period. [well done!]   I heard my parens [“When I was a child, I heard my parents say”] that you must not swim in the sea in [during] the Bon Festival. My mom [formal expression is better here ‘mother’] passed away 10 years ago.  I dreamt of about my mother Bon Festival. [‘I dreamt of my mother during Bon festival”].

Probably [here a comma is needed “,”] I think that she came to meet me [this sentence is correct, but it would sound more natural if you say “I think that she probably came to meet me”