Sandwiching of leave refers to the act of marking the holiday intervening two leaves as 'leave' instead of treating the same as holiday.
Whether a holiday coming in between two leaves should be treated as leave or holiday will depend upon the type of leave and the nature of industry. If a factory, the holidays intervening two 'earned leaves' should be treated as holiday and no way the said holiday should be sandwiched as leave. However, The Shops and Commercial Establishments Acts are silent about treatment of holidays intervening leaves. The principle is that leaves which are earned by working for certain days in the preceding year should be exclusive of holidays. At the same time, if you are sick on Saturday and Monday, it should be construed that you were sick on Sunday also. If you have a personal emergency on Saturday and Monday, let the Sunday be also on personal emergency.