foreach (GridViewRow rowItem in GridView1.Rows)
{
Button btnMonday = (Button)(GridView1.Rows[0].Cells[1].FindControl("btnMonday"));
btnMonday.Click += new EventHandler(this.mondae_Click);
Button btnTuesday = (Button)(GridView1.Rows[0].Cells[2].FindControl("btnMonday"));
btnTuesday.Click += new EventHandler(this.mondae_Click);
}
Because you do not use the 'rowItem variable in the loop, your code is now setting the same two buttons in Row[0] to have multiple copies of the same Event handler.