Iron, Bromine, Uranium, Argon, ?

Yttrium

The sequence, written in the character symbols for the chemical elements, is FE-BR-U-AR-

The sequence can be extended by the following:

Y (Yttrium), for FEBRUARY
I-ES (Iodine & Einsteinium), for FEBRUARIES (plural of February)
I-U-S (Iodine, Uranium & Sulphur), for FEBRUARIUS (Roman name for the month of February); and,
I (Iodine), for FEBRUARI (Dutch for February)

The best solution, therefore, is Yttrium.