Smaller doses don't really feel the same until you get high enough to have a proper trip. Taking that high of a dose would be more like moving your trip forward a couple days than trying in advance. Also, if you do take enough to feel much tonight, then you'll probably have a little tolerance which will make Sunday night less intense.
Feeling unprepared for your first trip is perfectly natural. It's a good sign, in fact, because it shows that you've done some research and know that it will be a pretty intense experience. Your best bet is to make sure you have a good set and setting and just dive in.
Depending on how potent your eighth is, you could probably get away with splitting it into 2 doses of 1.5-2g apiece. An eighth isn't a crazy high dose by any means, but it's on the higher end of the average dose range. Most people I know take between 2-4 grams on average.