Frequency adverb phrase
A frequency adverb phrase is an adverb phrase with a frequency adverb as its head, and which acts as a frequency adverbial. This may be a single-word phrase, in which case it’s simply a frequency adverb, e.g. "seldom", "sometimes", "often", etc.
These can be modified with an adverb, to produce multi-word phrases, e.g. "fairly often", "nearly always" and "very occasionally" as well as "twice-weekly", etc.