"You're invited to a party."
When children receive one of these invitations from their friends, they just can't wait. Usually they are ready days ahead of time. The "party" occupies their daily thoughts even thought it has not come yet. Their families are constantly reminded that the party is coming. They prepare by thinking of the appropriate gift as a response to its coming.
On the day of the party, they are dressed and ready well in advance. They keep checking the clock (or keep asking the time). As they finally prepare to leave, their excitement is tremendous. They never once think the invitation might be a "trick," because it is from the friend they personally know and have experienced.
Jesus asks us to be like children preparing for a party. Even though we have his personal invitation to come and celebrate eternally with Him, we don't yet know the exact time of his coming to pick us up. So we wait, watch, keep awake, anticipating him at any time with excitement and joy.