Experience has borne out that people are enthusiastic and excited when they are told the history of Hakhel, that in the time of the temple this year would be a Hakhel year when the people and the king would gather in the temple. Furthermore, in the spiritual sense the mitzva (commandment) may be fulfilled now, too, by attending a Hakhel gathering which will effect more fear of G-d and greater Jewish unity.
(From a letter of the Rebbe to Lubavitch Yeshiva of Manchester, 5748)