Hi
First - very sorry for your loss.
First option is to connect your mac directly to the TV. You can get a USBC to HDMI adapter for this, such as:
https://tinyurl.com/98jw82h4
(I don't know where you are located - so this link is from the Amazon UK store but you can just search for "USBC to HDMI adapter")
Two other options:
You can either stream the slideshow from your mac to a device such as an apple TV (or any device which can accept a screen share from a mac). This will require both your mac and streaming device to be on the same wifi network. Often a problem if the celebration is not at home.
Or you can export it as a video file. Many TV's can play video filed directly (eg from a USB Stick). If your TV cannot, then you'll need to connect another device to your TV that can play video files. This will be the best/most reliable solution if you have a way of playing video files on the TV
To export look top look for the export button top right of your slide show.
When is the event? Please get back if you need more help. I may be able to look up help for specific devices for you if needed.