singular or plural: [Thursday]

[ansonman]

I have made up the sentences below.

(1a) I sometimes work on Thursday.

(1b) I sometimes work on Thursdays.

Some of my non-native friends think the singular form of Thursday sounds better because the plural form works better with "always" and "usually".

Which form of Thursday is correct? Thank you.

[Alan]

Either 'on Thursdays' or 'on a Thursday'.