"Siku njema Esther"
Translation:Have a good day Esther
'Siku' is day, 'usiku' is night. 'Usiku' is a U-class noun which takes an m- agreement (or mw- before a vowel), so "have a good night" would be "usiku mwema."
Sidenote: 'siku' is also the plural of 'usiku' and takes an n- agreement, according to Wilson's Simplified Swahili. So 'siku njema' can mean both 'good nights' (plural) and 'good day.' But here, context still tells us it must be "have a good day."
The context is that "have a good day" is a standard greeting, while "have good nights" is not, at least in any culture that I am aware of. :)