You can't do that. You are free to choose which PH you want to observe e.g. CNY, Hari Raya, Deepavali, etc. If there is no one celebrating Wesak Day at your company, you can choose not to observe it as long as you choose other 6 optional PH. But you must observe the PH on the date it is declared, not some arbitrary day that you declare to be as a replacement for that PH.