There's a lot of sources on the internet which are happy to tell you that the Roman year originally had 10 months (Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December) and provide more-or-less authoritative derivations for those names.
However, nothing I've found explains why four (later six) months were named for gods (mostly) and six were simply numbered. Do we have any explanation for what happened to Quartilis, Tresember, Duober, or Unilis? (pardon the butchery)
Related, why are some of the numbered months -ber and some -ilis? Is this just Roman grammar, or is there some other reason?