Lots of festivities have been unfolding in New Orleans these past several days -- including elaborate parades that have been trotted up and down local streets there ... with some awesome floats on display.

There've been a handful of stars on the scene as well to celebrate Mardi Gras -- Neil Patrick Harris was spotted in NOLA on Monday ... and Kevin Dillon was out there on Sunday.

Yes, both of them both in the spirit of things ... with NPH in some fancy wardrobe, and KD dressed like a king.

There've been some other notable figures this week out and about to ring in the occasion ... like New Orleans Saints kicker Blake Grupe and Harry Connick Jr., who were both also seen on floats Monday night.

Like we said, Fat Tuesday is the tail end of the Carnival celebration in NOLA -- coming right before Ash Wednesday starts, and when people will presumably give up foods and crap.

Yes, there's some religious significance behind it -- even though many just treat it as an excuse to eat, drink and be merry in the streets.

That's certainly how some starts have gone about it over the years, anyway.