Upvote:2
The appropriate purpose is βtourism including visiting family and friendsβ or similar.
As to the date of the visit, thatβs up to you to decide providing the start date is within 3 months of the application date. Your parents can finalise the actual travel dates once the visa is approved, sticking more or less to the requested length of visit. UKVI specifically advise against pre-booking travel before you receive the decision; providing proof of flight/hotel reservations is not required when applying.