The current deadline to link your PAN (Permanent Account Number) with Aadhaar number is 30 June. If the deadline is missed, your PAN card will be treated as "inoperative" as per guidelines provided by the income tax department, previously.
You may also be aware than you can use Aadhaar in place of PAN if you do not possess the latter.
PAN is an important documentation/KYC requirement in financial transactions as well as income tax. Since not linking one's Aadhaar to the PAN card makes it inoperative, you may be charged a Rs 10,000 under Section 272B of the Income Tax Act for furnishing wrong or inoperative PAN in your records.
Note that even if PAN becomes inoperative, it will become operative as and when the linking process is complete. Since PAN is mandatory for certain banking and investments, it is better to complete the linking to avoid complications.
You can link your PAN with Aadhaar online on the income tax department's e-filing portal.