One thing ive always wonders is why when you say good night etc its nos but when you say tonight its heno
A word with very old origins, it seems. The -no part seems to have had a common origin with nos. The he- part is also found in heddiw (today = 'this day') and in the e- of eleni ('this year') - it is related to hwn, hon, hyn (this, these').
It saves us saying or writing y noson hon all the time, anyway!