If you make a contribution to your HSA between January 1, 2025 and April 15, 2025 you must tell the trustee or custodian of the HSA whether the contribution is for 2024 or 2025. If you don't notify the trustee or custodian that it's for 2024 they will assume it's for 2025.
Any contributions that you made from April 16, 2024 to December 31, 2024 are for 2024. Any contributions that you made from January 1, 2024 to April 15, 2024 are for 2024 unless you told the trustee or custodian that it was for 2023.
There is no "fiscal year" for HSA contributions.
