A reference to a weekend with no plans. A combination of the phrase free weekend.
Has nothing to do with sexual things which is a common guess by those who assume it is a "freakend" which is something completely different.
Often a freekend is something used to make plans to party.
or otherwise smoke marijuana.
"Dude do you wanna get together soon and toke it up?"
"Yeah man! i got a freekend comin up if you wanna make plans."