Every time there's a hardfork decision it's always tricky to know exactly if and when it will become active. There is a date and time that's programmed into the hard forks so that all the servers running them will switch over at the same time, but that doesn't actually mean that on that date and time people will be running them.
In order for a hardfork to pass you need 17 of 21 witnesses to approve it. So you either need 17 of the top 20, or 16 and 1 backup witness to sign off. To clarify to sign off in this case means you have to run the version of software on the witness box. In this case it's version 0.21.0. You can see who is running it now at steemdb.com/~witnesses. When we reach 17 votes in a 21 vote cycle we have a "super majority" and the chain hard forks.
As of today I was scrolling through and realized that we have 17 of the top 20 witnesses currently running HF21.
Does this mean that with 100% certainty that it'll go into effect on time? No!
Why not? Because witnesses can change. Right now 19 of the top 20 witnesses have votes from either 
There's also the newcomers with 
Lastly other whales can change votes, powerdown during the alt winter, or new witnesses can be voted up based on their general stance.
I'm a vocal supporter of this HF
HF21 has two parts the SPS and the EIP. The SPS stands for Steem Proposal System and diverts a portion of the rewards pool (currently valued around $500,000, five hundred thousand, dollars) into an account. The account can distribute this money to people who have submitted proposals and the community has supported them via stake weighted voting. The second part of it is the Economic Improvement Plan, which has several parts described below, but which is designed to change some of the voting patterns in the Steem Community.
EIP and SPS are both good.
SPS is good because it's an open process to anyone in the community to do any service on behalf of the community that the community finds valuable. This could be marketing efforts, programming efforts, community building, business building, or literally anything that can be turned into a post and voted on by the community. It makes the community less dependent on Steemit Inc and allows the community to dictate what work gets done based on the priorities of the stake holders.
EIP is good. EIP is comprised of 3 parts
- reward curve change that I think will have limited impact
- author:curator rewards will change from 75:25 to 50:50 which will make it more beneficial to hold Steem Power
- Free downvotes, which will help police the chain from circle jerks, stolen/unsited content, and generally unwholesome behaviors.
HF21 also comes with two smaller changes. Raising the custom json limit so that apps can submit larger transactions to the chain and also do it in such a way that they are less likely to fail. There's also a small change to when you can cast a vote and keep your full curation. Previously you had to wait 15 minutes. Now it's shrinking down to 1 5 minutes (you don't lose curation while voting after 1 5 min).
Hardfork goes live on August 27th
As all good hard forks should this one is going live on a tuesday! Assuming that the witness chart isn't truly scrambled between now and 9 days from now we'll likely see HF21 go through.
How to prepare?
If you're a backup witness you'll want to upgrade your witness. 
If you're not a witness consider making sure you have whatever crypto handy you're going to need in case anything goes poorly. It's unlikely the chain stops or any bullshit occurs with this one, but it can happen. Be sure to have some crypto handy in case you need it because of any interuption in service.
Be patient. It can take a little bit to get the bugs worked out, and witnesses might end up missing a lot of blocks when it first changes over. So, if you have big crypto moves to make you should probably avoid the 27th in particular.
Send good vibes. Let's all hope for a smooth transition to HF21. Send some good vibes for the chain and the price. New Steem is just over that ridge. We're almost there.