You can cast the video to your TV with AirPlay on iOS devices via the Safari browser. On Android devices, you can use the Chrome browser to cast to your Chromecast. Casting will not be available until the event begins. Below are some helpful links for your individual set up:
If you are not seeing the casting icon available, please make sure your Chromecast is on the same network as the device you are viewing the event. If that is the case, and you still do not see the casting icon, open your Google Home App, select your Chromecast from the device list, click the gear icon (settings), then tap the three dots in the upper right-hand corner and select "Reboot." Refresh the event, and you should be good to go.
Please note: Casting from the Chrome browser on iOS Devices to Chromecast is not possible due to Apple restrictions. AirPlay is recommended. Be sure to check that the device you are trying to AirPlay to supports AirPlay. More info HERE.