Lotusphere is the last full week of January (January 22-26). It is held primarily in the Dolphin, Swan, Yacht and Beach Club, and BoardWalk Conference Centers (in that order, in terms of heavily each facility is used), as well as some exterior space in front of the Swan and behind the Dolphin.
It has little to no impact on the park attendance. Every few years it will result in either MK or D-MGM closing an hour or two early.