I didn't write this but I thought it needed to be shared, original here

When is the next big sale going to start?

Nobody except for Steam's (Valve's) employees knows for sure. In past years, Steam has held two big sales per year: the Steam Summer Sale and the Steam Holiday Sale. In past years, the summer sales began on the last Thursday in June and the holiday sales began on the third Monday in December.
Therefore, in 2012, I expect the Steam Summer Sale to begin on Thursday, June 28th and the Steam Holiday Sale to begin on Monday, December 17th.
But, it is important to remember that this is just speculation. I could be wrong.
Also, Steam typically starts its sales (and changes its daily deals) at 10am Pacific time (Valve is located in Seattle).
How long do the big sales usually last?
The past two holiday sales lasted for 14 days and the last summer sale ran for 11 days.
What kinds of deals can I expect to see during the big sales?
See this comment.

Will [insert game title] be on sale during the big sale? If so, when? How much will it be?

Once again, nobody except for Steam's employees knows for sure. The rather vague answer to the first question is: probably. The price of almost all of the games on Steam get reduced for the duration of the big sales, often by 25% or 50%. Exceptions sometimes (but don't always) include games that were very recently released or ones that were very recently part of a smaller sale. Each day of the big sales, Steam picks a bunch of games to reduce even further, calling them the "Daily Deals". Nobody knows which games will get picked or for which days. Typically, these "daily deal" games get reduced by 50%, 60%, 75%, or in rare cases 80%. In past sales, the majority of the "daily deal" games seem to get the 75% discount.

Should I buy [insert game title]? Is the game worth it at this price?

Reddit is a good place to seek advice about the value of games Steam puts on sale. During the big sales, make it a habit to visit the subreddits /r/gaming, /r/games, /r/gamedeals, and /r/steamdeals. Within an hour or so of Steam posting the day's new sales, you'll find posts on those subreddits with titles like "Steam Holiday Sale 2011 Day 10".
These posts will contain information about the current sales, current prices, and whether the games are worth buying. There is usually a pretty good discussion in the comments about the pros and cons of the games, as well as comparisons with past price cuts or sales. If you don't see a mention of the game in which you're interested, you can post a comment asking about it. But often if you just wait a bit you'll start to see some detailed reviews and comments trickle in. Another quick way to get opinions on a game is to post to the subreddit /r/ShouldIbuythisgame.
Steam also tends to add secret, un-advertised sales each day. Redditors are pretty good at finding these deals and information is often provided in the same posts.

Why are we supposed to wait until the last day of the sale to buy games that aren't daily deals?
During the big sales, Steam discounts just about every game for the entire length of the sale. Each day, Steam takes a handful of those discounted games and discounts them even further, calling them the "Daily Deals". So a game that was 25% off during the first three days of the sale might become a daily deal on day 4 and get marked down to 75% off. On day 5, it goes back to 25% off and stays that way until the end of the sale.
Not every game becomes a daily deal. So if you've got your eye on, say, Civilization V at 50% off, you should wait to see if it becomes a daily deal and maybe get 75% off instead. If it never becomes a daily deal, then on the last day of the sale, you can still at least pick it up for 50% off because you won't see at better deal at that point.

How will I know when the Steam sale will end?

Once the sales are live, Steam will usually list the end date quite prominently.

I'm not at my computer/don't have a computer, can I still buy games during the sale and download them later?

Yes! With Steam, once you purchase games (which can be done from Steam's web site, the Steam client, or Steam's mobile apps), they are added to your account and you can download them whenever, wherever. You can even tell Steam to start downloading games to your PC remotely.
If you don't have a PC yet, you can still purchase games now and access them on your new PC later. You will always be able to access your games as long as you remember your Steam username and password. If you have Steam Guard enabled, however, you may need access to your email to input the security code when you try to log in to the store.

How can I avoid problems with multiple small charges to my credit card during the sale? What if I don't have a credit card?

If you plan to make a bunch of purchases from your credit card, add some money to your Steam Wallet once using this link and pay for games from that, so your credit card doesn't get falsely flagged by fraud detection.
If you don't have a credit card, you can purchase Steam Wallet Code cards from Gamestop.

What is this AU USD or AUS that I see listed in the various posts on reddit?
It refers to the price of a game in Steam's Australian store, which charges in US dollars.
Are complete packs ever part of the daily deals?
Yes, complete packs are often part of the daily deals. For example, during the last sale, the Grand Theft Auto Complete Pack was on sale for about $12.50 USD (75% off).

If I buy a package that contains some games I already have, do I get extra copies?
Most likely not. Read here.
How do publisher pack sales work?
For the entire duration of the big Steam sales, a number of game publishers bundle all (or almost all) of the games they publish together into a package that you can purchase for one price. The package price is much less than the regular cost of the games if you bought them all individually and is even less than the sale price of the games if you bought them all individually.
For example, if you individually bought all the games sold on Steam by publisher PopCap, it would cost you about $350 USD. Even when there isn't a big sale, you can buy a package of all these games at a substantial discount: about $100 USD for the PopCap Complete Pack.
During the last big sale (the Steam Holiday Sale 2011), most of the games in the package were on sale individually. Even then, if you bought all of them individually they might have cost about $100 USD or more. However, during the sale, the PopCap Complete Pack was available for $50 USD, a fantastic buy.
A word of warning: the publisher packs (or any other packs on Steam) are not always less than the cost of the games if bought individually. This is more frequently true when some (or all) of the games in the package are on sale themselves. Fortunately, at the bottom of the pack pages Steam always lists the total price of all the games if bought individually right above the cost of the package. If the package price is less, you'll see in green text, Here's what you save by buying this bundle along with the savings amount. That green text won't be there if the package price is higher.
To get an idea of the kinds of publisher packs available during the big sales, check out this list of the ones that were available during the last sale.

Do games sold on Steam employ Digital Rights Management (DRM)?

Yes. Almost all games you purchase on Steam require the Steam client application to run. (Some games, such as a few published by Paradox, require Steam only for downloading.) However, many people find the various features of the Steam client application so useful that they are willing to accept this minimal form of DRM.
Some games on Steam employ DRM schemes in addition to Steam. For example, Crysis employs SecuROM on top of Steam's DRM scheme. Extra DRM such as this is sometimes referred to as "3rd party DRM". A comprehensive list of games with 3rd party DRM is available at The Big List of 3rd Party DRM on Steam.