I have this one site which in the last month has had a day where it earned $2 and a day where it earned $30 (yeah, I wish I had $30 every day) - crazy really! Adsense earnings can really make no sense from day to day - apart from variations in traffic, you get big variations in CTR and also in CPC. Who knows why? You can't always know. Sometimes it's probably major advertisers starting or stopping campaigns and the most relevant ads either appearing on or disappearing from your site (affecting CTR), or a little bidding war with advertisers ramping CPC up or down. Sometimes it may even be delayed reporting by Google. The best you can really hope for is to get a consistent MONTHLY income (though things can vary quite a lot even from month to month).
I say all this because when people start out they set themselves a kind of target like $5 a day, or something, but in reality it is rare to get consistent earnings like that with Adsense (or pretty much any other business really).
Great, of course you should set targets for yourself, but I would look at a monthly target rather than a daily one, and also not get TOO hung up on that goal. Adsense is kind of a numbers game, and you really want to be building out that site, concentrating on the content, and on what kind of subject areas you can cover within your chosen topic. Yes, ranking for keywords, but taking a wider view, writing for visitors and watching Google send you tons of long tail traffic and slowly build you up as an authority.
Yes, I guess I am advocating (like many others) a kind of authority model where you try to be the best you can in your chosen area, that is a little wider than just "x keyword phrase with 6000 searches per month". Fine if you end up ranking for that KWP, but don't think that ranking for that single keyword will necessarily bring you that stable income of $5 a day, or whatever. Always be thinking about how you can leverage your site for even bigger returns, don't agonise over each individual click and your day-to-day earnings (or lack of them) and watch that balance at the end of the month grow!