I can't tell if this is a joke suggestion or not, but if it isn't, there's way too many reasons not to do this. There's confusion among new players, the deviation away from the gamemode that people are actually there for (they can just join a DM server if they want that), the absolute nightmare that will be dealing with reports after that ("oh it was a FFA round"), and overall just an unenjoyable experience for those on the server (there's more reasons that I won't bother typing out on my phone) – if people want DM, they should join a DM server. TTT is, by its very design, not DM. It'd be comparable to saying that 1 day every X time, everyone on GTA can just spray anyone they see down without RP, it just blatantly goes against the very core of the gamemode. We should stop trying to cram gamemodes that have zero relation to the actual server in as "special rounds".
Huge -1 from me. Unless there is an extremely overwhelming amount of support for this suggestion (including from regulars on TTT), I do not believe this should be implemented.