China is notorious for having much heavier tourism during public holidays. I would expect much more people than the other days. Here is an image from the 2012 labor day flag raising:
This here is from the forbidden city 30th April 2012:
And this here from the great wall May 1st:
I would therefore strongly advise you NOT to go there before 4th of May. You will have a much better experience and waste much less time waiting longer in line than actually seeing something.
I do not think sure the flag raising will be different that day though, it should take only 15 minutes, as you said
There is the forbidden city right next to it, but it opens 8:30. The large building in the background of the first image is the entrance to the city. The flag raising happens right in front of that. I would not be surprised if people would start lining up for entrance to the forbidden city right after the flag raising since it will take you more than 1.5 hours to get in anyhow during labor day. On a normal weekday, it should be a much shorter wait, specially if you arrive early.
Jingshan park opens at 6:00, so that might be the best option to visit first. On normal days you can see people practicing Tai Chi, social dance and lots of other stuff there, which is actually the best thing about this park. There are people practicing calligraphy with water on the stones, they might write your country name for you. If you stand there and take a picture, they are likely to start talking to you (but most likely in Chinese). I had a really great time there and the photo opportunities are all over the place. On labor day, if there is a major tourism flow, this might be even tricky since the local people might avoid the place too.
If you want to go to Mao's mausoleum, it opens at 8:00 am, please be aware that shorts or slippers are a no-go (but I guess early morning in May might be too cold for that anyhow). But more importantly, you cannot take bags or cameras inside. Photos are not allowed. If you do not have a place to store these (Car, tourguide etc), you are not able to go in! The lines in front of that even on a normal weekday will be at least 30 minutes. You can imagine how long it will take on labor day. Please note however that this is a walk-through visit. This speeds the line up to walking speed, but you are also not allowed to stop in front of the coffin. You walk past it in the crowd.
So the best schedule might be:
- Flag raising 6:00 - 6:20 (come 20 minutes early to get a a front-line spot)
- Jingshan Park (walking takes about 40 minutes) from 7:00 to 8:00 (plus walk back)
- Mao's Mausoleum (let's assume 30 minutes wait) 8:40 to 9:15
- Forbidden City (if you take your time, this can take 2-3 hours)
Of course, if you can catch a taxi instead of walking, you can be in most places more or less just when they open. Since you walk from Tiananmen square to the park and back to the same place, a taxi might be the better choice anyhow. There are multiple cafes and restaurants in the forbidden city to have some late breakfast. Around Tiananmen square and in the park this is a bit harder to find.