As former Jehovah's Witnesses we were required to attend large conventions every year at our own expense. During the year, smaller assemblies were held on weekends. These events brought together Jehovah's Witnesses from local congregations. If you're curious as to what goes on at conventions, this is what we observed.
Typically once per year Jehovah's Witnesses look forward to attending a large convention, where several thousand people gather to learn 'new truths' and words of wisdom from Jehovah God himself, as explained by the Watchtower Society. Also at this time, new publications, books, pamphlets, brochures, and booklets are released. Jehovah's Witnesses eagerly rush to buy this literature the moment it is available.
Jehovah's Witnesses are programmed to accept new beliefs as the 'truth' from Jehovah. Old doctrines, mistakes, and false prophecies are quickly swept under the carpet and quickly forgotten. Jehovah's Witnesses are kept so busy learning new 'truths' from the Watchtower, they don't have time to examine old Watchtower literature for truthfulness. 
New 'truths' are also called 'new light'. Jehovah's Witnesses believe that's Jehovah's light get's brighter and brighter, as he explains his will to his 'slave', i.e. the Watchtower's governing body. 
Of course what Jehovah's Witnesses learn at conventions and assemblies is not from Jehovah God, or Jesus. It's the Watchtower's interpretation of their faulty Bible at this point in time. What Jehovah's Witnesses believe to be 'the truth' is merely the opinions of the governing body at this point in time. We know this because the organization and governing body does not claim to have any divine inspiration.
In between large conventions, Jehovah's Witnesses often enjoy smaller assemblies where perhaps people in their 'circuit' or region will gather, typically on a weekend for a couple of days. Sometimes it's just a gathering of local congregations combined into one meeting. At these smaller conventions, Jehovah's Witnesses meet to strengthen their faith and learn how to be more effective spreading the 'good news' from door to door.
When is the next convention? Jehovah's Witnesses zealously promote and advertise these special events months in advance. If you want to attend the next convention or assembly, contact your local Kingdom Hall of Jehovah's Witnesses.
New recruits, weak Jehovah's Witnesses, and all interested people are encouraged to attend. The excitement of meeting in large groups and seeing so many 'like minded' people 'speaking in agreement' is a powerful recruiting tool for people who don't understand why Jehovah's Witnesses agree on all doctrines.
Also since most Jehovah's Witnesses have nagging doubts, large assemblies and conventions are an encouragement for members to continue serving Jehovah by being loyal to their organization. 
1. Jehovah's Witnesses rarely have time to practice what they preach about 'making sure of all things'. And there is no incentive for a faithful Jehovah's Witness to 'examine their religion' unless they intend to leave. Questioning doctrines, is considered 'running ahead of Jehovah' and the beginnings of apostasy. Expressing doubts even to family members and close friends is dangerous territory. There's always the fear someone, perhaps even a loved one will report you to the elders. Spreading 'rumors', grumbling, disagreeing, and 'politicking' is not allowed within the organization.
Jehovah's Witnesses pride themselves on being 'like-minded' and 'speaking in agreement'. Their primary focus is unity of beliefs and the interests of the organization. Individualism and 'independent thinking' is discouraged. Therefore Jehovah's Witnesses will defend all the beliefs and interests of the organization even when they secretly disagree or have serious doubts. See Avoid Independent Thinking
Disobedience to 'Jehovah's' organization can result in disfellowshipping, which is a very serious matter. The risk of losing their family and closest relationships is too great for some Jehovah's Witnesses. Therefore it may take years to gather up enough courage and determination to leave the organization. See Seven Reasons Why It's Hard To Leave
3. One the surface, the Watchtower and Jehovah's Witnesses seem to be about God, morality, love, and 'the truth'. However critics say the Jehovah's Witnesses are a cult. The Watchtower requires JW's to refuse blood transfusion to save a life. In addition, Jehovah's Witnesses must have absolutely nothing to do with family members and close friends who leave the organization (except for limited exceptions). See the Watchtower's Shunning Policy