To create a custom header on a group, you can use the criteria if you enclose it in curly braces. In the following example, I have a group header custom text defined for Group 0 which will display the criteria named location and wednesday

{location}
{wednesday}

Additionally, you can define a style for the custom header by wrapping it in curly braces also, and placing it after the custom header text. To center and bold the above custom header, use:

{location}
{wednesday}
{STYLE text-align:center; font-weight:bold;}