Northbound Hikers: typically leave Georgia late February through April and begin reaching Katahdin in late July through September. Owing to declining weather, thru hikers must reach the northern terminus by October 15 in order for a realistic chance to summit Katahdin as access thereafter is rare to non-exisitent.
Southbound Hikers: typically start from Katahdin June through August and begin to reach Springer in October through early December. There is no hard wall for reaching Springer as there is for Katahdin. However, the bulk of Southbound Hikers arrive mid November through early December, and portions of the Trail (particularly in the Smokies) can become impassible at high altitudes as early as November in some years — and not at all in others.
Flip-flop Hikers: (i.e. hikers completing the Trail in discontinuous sections) typically leave from an approximate mid-point heading north (Rockfish Gap VA, Harpers Ferry WV, and Delaware Water Gap PA being popular starting points) in May through mid June, arriving with the bulk of Northbounders at Katahdin late August through September. After summiting Katahdin, most, flip-flop hikers return to their mid-point and hike south towards Springer, typically arriving with the bulk of the Southbounders in late October through early December.
— from the Appalachian Trail Database