I have a 2 year lease agreement that is very vague. It states that I pay $1900 a month for 2 years. It doesn't state when the lease payment is due. My landlord tried to serve me a 3 day pay or quit notice cause I still owed $400.00 for the lease payment. I told her no. what are my rights on this matter.

In the absence of a specific due date, your rent is due on the day of the month when the lease was effective (i.e. the day you moved in or took possession). So if you moved in on the 1st the rent is due on the first.

Unless the lease specifies a grace period in is late after close of business on that day.