What is Tender Period?

It is the period, that starts 5 days before the Contract expiry.  

Navia Policy 

Navia shall not allow the client to mark delivery of open positions of a future commodity contract and the open positions shall have to be closed out before the tender period starts. If at any time an open position of a future contract is not closed out on or before the expiry date by the client, depending on the long/short position, Navia shall close out all the open positions, and the client shall be fully liable to indemnify Navia, for any loss/losses that may occur due to the close out of the positions.

Why is an additional margin charged for Crude Oil & Natural Gas contracts close to expiry?

MCX (Multi Commodity Exchange) collects an additional margin on Crude Oil & Natural Gas contracts on the last 5 trading days before expiry (called “tender period”). The additional margin will be applicable from April 2021 expiry onwards as per this exchange circular.

The additional margin amount (also called the pre-expiry or tender margin) is blocked from your funds 

For instance, the expiry day for the Crude Oil contract is on the 15^th of a given month. The additional tender margin will be levied as a percentage of the contract value in the following manner:

The additional tender margin on commodities is charged to cover the risk of large price fluctuations in commodity contracts that can happen close to expiry. 

You can also check out this circular on SEBI's Alternate Risk Management Framework to know more about tender or pre-expiry margins.