Because most adverbs end in ly, some people mistakenly add ly to the adverb seldom.

Seldom is both an adverb and an adjective. Seldomly is non-standard English.

Use seldom

• as an adverb
  

  Ontarians seldom (not seldomly) have such warm weather in November.

• as an adjective

  A weekend without a drop of rain is a seldom occurrence in Vancouver.

  Note: The use of seldom as an adjective is rare. Consider using a synonym instead. For example:

  A weekend without a drop of rain is an uncommon occurrence in Vancouver.