Our vacation starts on 17may.
The annual function will held on 17 may.
In both sentences the event is scheduled. But we are using diffent tense i.e. present in ist case and future in second case, WHY?

Simple present tense is used to talk of a scheduled event in the near future. For example: 'Our vacation starts on 17 May.'

Simple future tense is used to express a firm plan. For example: 'The annual function will be held on 17 May.'