* Major indexes posted their biggest one-day jump of the year on Wednesday in a broad advance after the Federal Reserve reassured investors its first rate hike would come when economic data pointed to an economy that could grow without Fed stimulus, rather than on a specific schedule. Any follow-through to the rise could be a sign that momentum has swung to the upside, and investors will be looking for the S&P to break above its 14-day and 50-day moving averages.