The simpliest thing I coud do is just to paste a basic statement from the www.dayofsilence.org
website that sums up the purpose of the day.
DAY OF SILENCE, April 25, 2008: The Day of Silence is an annual event held to bring attention to anti-LGBT bullying, harassment and discrimination in schools. Students and teachers nationwide will observe the day in silence to echo the silence that LGBT and ally students face everyday. The Day of Silence is one of the largest student-led actions in the country.
A good idea we thought of but weren't able to do on our first year doing this was to put up posters around the school with national statistics concerning LGBT bullying, violence, etc. in school. At lot of time people just aren't aware of the discrimination that exists, so education and awareness are some simple ways we can fight that ignorance.
Often times I've had friends ask why take a vow of silence or how is that helping, and of course I can't answer, but I tell them the next day that it's supposed to symbolically represented the literal silence of GLBT students everywhere, though bullying, and unwelcoming environment, etc. I also explain that it is supposed to raise awareness of that enviroment. We thought the posters would be a good idea because it will raise awareness of the scale on which what we're protesting is actually happening.
We had a breakout time, an end of the Day of Silence (I forgot what we called it) type event after school let out, and we shared our stories from the day, talked about how we could improve it next year, etc.
The Day of Silence website has a lot more information about how to organize the event, order material for the event, and get help in organizing the Day at your school.
Heh, I ended up saying a lot more than I meant to, but hopefully it was very informative.