It comes from Teaching Unplugged by Luke Meddings and Scott Thornbury. It's quite simple and when I first came across the activity I was feeling something close to regret I hadn't thought of it myself. It's a very sincere and authentic task,as it seems to me, and it lets you create something (almost)any student can relate to. A nice - probaly the best - way to start a lesson and to set positive mood for you and your students.
Here is what you should do.
First, do the task yourself: draw a graph of your mood (during today's moring, during the whole day yesterday, the whole week, etc). You can add a bit of an intrigue here and ask your students if they know what the graph is showing.Chances are, they won't quess correctly, but it's ok.
Then tell that this graph represents your mood of yesterday (or any other period of time). Here you can tell all the truth about your mood and reasons for it, but there is no reason why you might not like to color the truth a little. Don't hurry - take your time, be emotional, share how terrible you felt at 5:45 a.m. when you had to wake up, or how happy you were when buying those great shoes..and then ask your students to draw their mood graphs too. Here you have to be perstistent, as many of the students (at least that was my experience) tend to skip the drawing part and just to tell you about their mood - who needs drawing?. Thus, a meaningful half of the experience (remembering, putting it on paper, drawing)will be lost, so I'd recommend they still draw - otherwise it will be just the usual telling-about-your-mood.
After the drawing part is done, they then tell you (or their pair, if it's pairwork) why exactly they felt that way, and what happened to make their mood line go up and reach all-time high again/ drop so suddenly, etc.
Being a nice starter, it is also a good way to practise verbs of change, as well as grammar tenses. You can also make the activity more dynamic and conversational by offering students to ask each other questions about their mood changes.
For me, this activity has always worked: with both German and English, and with different levels, from low pre-intermediate and up. And has always helped me to generate that priceless emotional response. Oh, and I suppose the fact that you let your students in on something as personal as your mood also plays an important role in student-teacher relationships building.