Sounds like you can’t submit your proxy on the day of the meeting. If you could it would say “first meeting … held on or after the date that the authorisation is delivered.”
It sounds petty not to allow an authorisation to be delivered at the start of a meeting, but delivering it the day before possibly gives the Secretary time to check that the proxies are valid and to address any issues. I’ve had to deal with on-the-spot proxies in a non-strata AGM where 200 people turned up and about 50 were on-the-spot proxies. It was chaos.