I believe it is called Le Club Accor day rather than Accor Day. Supposed to be day to mark the Accor loyalty programme.
My only experience of it actually being marked was Hotel Muse Bangkok which opened its evening drinks and canapés to all Le Club members staying in the hotel on the day. As a Platinum card member the only thing this achieved was making evening drinks and canapés (which was a daily Platinum benefit in lieu of a full lounge) busier that night than usual. The hotel also offered Le Club members a lucky dip for things like a 25% discount voucher on F & B, free airport transfer etc.
I understand it is left to individual hotels to do whatever they wish or do nothing. Managers drinks receptions I believe are a popular hotel choice to mark the day but as with Accor generally Asian hotels are better at this sort of thing compared to their European cousins.
If you're staying in a Accor hotel on the day adds a little to the stay but not sure its worth changing plans on booking specifically to be in an Accor hotel on this particular day.