Please, don't do it like that!
Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
The chances are that that will also cure your problem.
string query = "INSERT INTO Events (event_name,event_descrption,event_type,date) VALUES(@NM, @ED, @ET, @DT);";
try
{
conn.Open();
SqlCeCommand command = new SqlCeCommand(query, conn);
command.Parameters.AddWithValue("@NM", textBox1.Text);
command.Parameters.AddWithValue("@ED", textBox2.Text);
command.Parameters.AddWithValue("@ET", comboBox1.SelectedItem);
command.Parameters.AddWithValue("@DT", dateTimePicker2.Value);
command.ExecuteNonQuery();