What do charismatic Christians actually mean by the phrase "tonight is an open heaven"? I have heard this phrase bandied around for about the last 10-15 years and I am wondering how it originated and if it has any scriptural basis.

My own belief is that in the light of the cross and the finished work of Christ, it is always an open heaven and God is always accessible everywhere.
It's a term used to describe  the revelation of something the Holy Spirit showed them in a dream or a word of knowledge.