Using "seldomly"

Contrary to your (very reasonable) gut feeling, seldom is an adverb, even though it doesn't end in -ly.

Teachers will tell you to use seldom, which is an adverb already so doesn't need -ly. But Google Books will tell you people have been using seldomly since the eighteenth century and still use it today — though considerably less than seldom.

So the best advice (in my opinion) is to use seldom. But seldomly is not wrong.

As already pointed out, the word "seldom" is mainly an adverb, even though it can be used as an adjective in some cases. And although "seldom" may be seldom in every day language, "seldomly" occurs even more seldom.

This stands in contrast to the word "often", which in relation to "seldom" is used very often.

Note that both "seldom" and "often" are adverbs, but that none of them ends in "-ly" which adverbs almost always do otherwise.

In all honesty,the word 'seldomly' is not acceptable in contemporary British English.Attaching the suffix -ly to 'seldom' to create an adverb,in my scholarly opinion,constitutes a redundancy of the highest order,simply because 'seldom' is already an adverb.