Rental lease is up 3/31. They want me to pay an increase of $100.00 starting the very next day 4/1. Do they have to give notice?

Read your lease. Many say that increases are at landlord's discretion.

In Seattle, a rent increase of 10% or less must be accompanied by a 30-day notice and a bigger rent increase must be accompanied by a 60-day notice. There is no rent control anywhere in WA. Your current written agreement may already have a provision for a rent increase if you do not move out when your lease is up. You should review the specific facts with your attorney to find out your legal options.

I agree with the previous answer and you are entitled to at least thirty days notice even as a month to month tenant whose lease expired . Good Luck

It depends on what the lease says. If your tenancy turns into a month to month upon expiration of the lease then the landlord must give you 30 days notice if the increase is 10% or less and 60 days notice if more than 10%. If the lease is silent about whether it turns into a month to month then it simply expires on March 31 and neither party has to give any notice about moving out or not renewing. In that case, the old terms are gone both you and the landlord are free to agree to new terms. It is like you are making an agreement for the first time and it is up to you to agree or not agree with those terms. It's pretty common for the rent to increase each time you sign a new lease, but $100 seems like a lot. You may want to try to negotiate some upgrades for that price.