Execute an event when a specific button inside a gridview is clicked

foreach (GridViewRow rowItem in GridView1.Rows)
{
    // why are you never using the 'rowItem variable ?
    Button btnMonday = (Button)(GridView1.Rows[0].Cells[1].FindControl("btnMonday"));
    btnMonday.Click += new EventHandler(this.mondae_Click);
 
    // why is the Button in Cell[2] named 'btnMonday' ?
    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.

for each button in the command name add different value
& in the design make the click event button for the all buttons is the same event

then in the event click try this code

switch (((button)sender).commandname)
{
case your value
execute code
break;
case your other value
execute code
break;
}