The salary has to be paid on its fixed/due date/usual pay day.T
The employee can lodge a complaint the moment payment of earned wages is delayed even by a day and employer can be penalized by say Rs.7500/instance.
There is no law that due to year end closing payment of salary can be delayed and also that if employee has resigned salary of last month (Mar) can be withheld. Even April salary has to be paid by usual pay day.
The FnF payouts have to be paid on last day in office/within next 3 days or max. by usual pay day.
Since you have tendered notice of resignation employer has sufficient time to process FnF and complete all of its internal formalities and handover all payouts /relieving docs on last day in office.
You should submit a formal communication addressed to good offices of appointing authority/MD,under proper acknowledgment narrating all representations for non payment of April salary and supply you the salary slip of April with April salary immediately and to supply the FnF payouts/all docs on last day in office.