Specifying Non-Termination Months in Month-To-Month Lease?

[Josh Tsang]

I have seen residential landlords with month-to-month leases that specify tenants cannot terminate their leases during specific months of the year (i.e. November - January) as to avoid vacancy and headaches during periods that are usually more difficult to find tenants. Do any of you know if such language is legal and enforceable and allowed in Washington state? If it's questionable, or un-enforceable, could such language make an entire lease null and void?  Thank you!