Date of deduction is the date on which TDS is deducted by the payer/ deductor while making payment to payee/ deductee. It needs to be mentioned in Payment/ Deductee Details. Usually, it is the same as date of payment.
For example, XYZ Pvt Ltd made payment of Rs. 50,000 as rent on 15/04/2020 then the date of deduction (TDS) should be 15/04/2020.
If the date of deduction is after the date of payment then penalty interest is levied u/s 201(1A) on payer/ deductor for Late Deduction. Interest is 1% per month from the date on which tax was deductible to the date on which such tax is actually deducted.