These extra lessons are given for those who will turn seventeen on or before April 21st, which is the date of the first Apparition test. Ron and Hermione both go into Hogsmeade for the lessons while Harry, who won’t turn seventeen until July, spends his time trying to figure out why Draco Malfoy is spending so much time in the Room of Requirement (HBP21).
Timeline Notes
The practice sessions take place on a Sunday two weeks before April 21, which we'll make April 7th based on the actual calendar of 1997:
And so the following weekend, Ron joined Hermione and the rest of the sixth years who would turn seventeen in time to take the test in a fortnight ... As it was Sunday morning, nearly all the students were inside their various common rooms, the Gryffindors in one tower, the Ravenclaws in another, the Slytherins in the dungeons, and the Hufflepuffs in the basement near the kitchens (HBP21).