To refer to a control on another form just use the form name and a . before the control name;

eg;

FormName.TextBox1.Text = "Hello"

so, in your example, the FormA button click event should be;

Sub CommandButton1_Click
FormB.TextBox1.Text = Calendar1.Value
End Sub

I've never used the calendar control so I'm not convinced .Value is the right property - if it isn't just change it to the appropriate property to retrieve the date.