You can cancel a travel day only before it starts. You should only activate a travel day when you're sure you're going to use it, because if you change plans and forget to cancel you travel day, then you loose it.

You can ask customer service if you can get your travel day back but that can take a while.

Thank you for your reply. Although I have already asked my issue to customer service, I have not received response anything.

Did you mention to customer service when you need your lost travel day? Because that wil help to prioritise.

I would like to use my lost ticket 24th of May. I hope you help me.