Siwāk is from the Sunan that may be done at anytime. The Prophet (PBUH) persistently urged us to use it.
Anass (ABPWH) narrated that the Prophet (PBUH) said:
“I have indeed urged you regarding Siwāk.”
(Al-Bukhāriy no. 888)
‘Ā’ishah, may Allāh be pleased with her,
narrated that the Prophet (PBUH) said:
“Siwāk cleanses the mouth and pleases the Lord.”
(Ahmad no. 7 and An-Nasā’iy no. 5)
Using Siwāk is most recommended in certain situations such as upon night prayer, upon Wudū’, before every prayer, and on entering one’s home, and Allāh knows best.