Tthe circumstances of my all my pregnancy timing were bad in other people's eyes, so we waited longer for different reasons with each pregnancy.
We waited until around the 10th week with my first to tell several close friends (namely the wedding party and our pastor). I was showing by then and it was getting really obvious, anyway. My parents didn't know until after that. I had to tell my work at 9 weeks when I was put on a day of bedrest.
With the second, my sister figured it out quickly, and I think we told friends/family before 8 weeks.
#4, told husband when I took the test, about 4 weeks and family about 6 weeks, when I had to take it easy for a week. I think church/close friends knew about then, too. Didn't make it to announcing to anyone else.
With the 3rd, I told my now closest friend at about 4 weeks, but I hadn't had a test to know for sure. I told my husband after the positive test around 6 weeks, and at 9 weeks m/c. I'd planned to wait until 12 weeks, which was when I wanted to hear the heart for the first time. At 13 weeks, still growing and tired, but thinking something was terribly wrong I found out I was still pg, so I just had to tell the whole world.

I like the suggestion of not giving the reason for changing the wedding date, just making a general announcement that the day you had chosen won't work due to scheduling conflicts. People will think all different reasons, anyway.
I fully understand wanting to wait a certain amount of time before announcing. I've heard it's good to wait until the beginning of the 2nd trimester to announce to work.
There's no such thing as global warming. Chuck Norris was cold so he turned up the sun.