Today, nothing definitive happened in Greece. "Stocks rallied for a second day on Thursday as Greece backed away from a proposed referendum that threatened its membership in the euro, which could destabilize global markets." So - stocks were strong today because the Greeks decided to not do something that maybe they weren't going to do anyway, and nobody knows what the effects might have been if they had. Well, stock increases climb a wall of fear and uncertainty, so that all makes very good sense.
Another way to look at it is that after a five wave decline covering 3 trading days, the markets are now in a corrective phase. From Thursday's SP500 high of 1292.66, the index bottomed at 1215.42 on Tuesday. Today's high of 1263.21 is only .06 above the .618 retracement level. Of course, there is no particular reason for the advance to stop exactly there. Recent retracement have been quite steep and the .786 level, at 1276.14, is not far off. The way markets move these days, that could be reached in a few seconds of trading.
If the advance were to end now, it would mean that, once again, it faltered at its trend midline. That would be esthetically pleasing, at least. Note also that the open gap from Tuesday's opening has been filled.
If the index doesn't turn down here, I expect it will soon.