You can show the bubble using a time header active area:
onBeforeTimeHeaderRender: args => {
const header = args.header;
args.header.areas = [
{
left: 0,
right: 0,
top: 0,
bottom: 0,
action: "Bubble",
bubble: new DayPilot.Bubble({
onLoad: args => {
const start = header.start;
args.html = `${start}`;
}
})
}
];
}