Will it be safe to have a barbecue on Fourth of July with my extended family, maybe 20 people?

In most places, if you weren't able to do so, it means things are really bad. My answer is a guarded "yes". I'm neither pessimist nor optimist, and just looking at it pragmatically.

Maybe,  There are too many variables involved.  If I were you, I would plan it, for outdoors.  I would also be ready to cancel it.  

Variables: How many have had the virus.  How many have been tested?  Are the old people vulnerable because they don't have antibodies and have not had the virus. Do the kids have it or their friends at the party.  How much of a risk to you want to take with your family?  On the bright side, the virus should slow down because people are not stuck in their houses due to the weather.  So its possible.  

Of course not. We have a pandemic until there is a vaccine next year. 

Happy fourth of July. 

Almost certainly not.  

Until there's an efficient vaccine available and we've all had it, no-one is safe.