how to write validation code for checkbox

i'm newbie please help me, and i have datatable

    unitid   date(textbox)       checkbox
    id2323  01/01/2019(input)     check
    id2323  01/01/2019(input)     check
    id2323  01/02/2019(input)
    id2324  01/01/2019(input)

when saved, show the message
lblMessage.Text = "date cannot be the same";

        unitid   date(textbox)       checkbox
        id2323  01/01/2019(input)     check
        id2323  01/01/2019(input)     
        id2323  01/02/2019(input)     check
        id2324  01/01/2019(input)

when saved, show the message   
lblMessage.Text = "save succesfull"; 

thanks