missing first day of university term

so basically i start my course this month but i have a pre booked event. essentially a few months ago my best friend surprised me with rlly expensive vip tickets for an event to meet my favourite actors and the uni website originally said term started the day after the event but it suddenly changed so now it’s my first day of term. id still be moving in on the 15th and therefore making friends, my induction week is the week after i move in too and before the event and ill be attending every day of that too. ill only miss the one day which is the 6th of october and i only have an online tutorial and an introduction briefing. is this bad? i was thinking about what to tell my lecturer and would it be better to tell him the truth or just say i was sick? either way the tickets are non refundable and very expensive (i’m too scared to check how much she spent on me haha) so i can’t really miss it.