Times when it is Sunnah or Mustahabb to take ghusl

It is a sunnah to take Ghusl for:

1. Jumu^ah (Friday) Prayer
2. The 2 Eid prayers
3. The Ihram (proscription - entering the sacred state) of Hajj Pilgrimage
4. The pilgrim in the place of ^Arafah on the day of ^Arafah, after the sun's zenith.

It is Mustahabb to take Ghusl (Full Body Wash) for:

1. The one who became muslim and was Tahir (otherwise he must take ghusl).
2. The one who reached puberty by age.
3. The one who became sane after being insane.
4. The one who did hijaamah (cupping with small cuts to let some blood out).
5. The one who washed a dead person.
6. The night of the middle of Sha^baan.
7. The night of Qadr, if he saw it or according to narrated hadiths.
8. Entering Mediinah (the Prophet's City - may Allah raise his rank).
9. Staying in Muzdalifah in the morning of Eid in Hajj.
10. Entering Makkah for Tawaaf of Ziaarah (the obligatory one in Hajj).
11. The prayer of the eclipse of the sun or moon.
12. The prayer of asking for rain.
13. A prayer performed due to fright.
14. A darkness that happened during the day.
15. When there is very strong wind.
16. Repenting from sins.
17. The one who returned from travel.
18. The one who is wanted killed.
19. The woman who's irregular bleeding stopped.
20. For throwing pebbles in Hajj.
21. For wearing new clothes.
22. For going to a gathering of people.
23. The one who came in contact with something islamically filthy (najis), but does not where it is on his body. This is also true for his clothing in this case - it is better to wash all of it.

Source: "al-Maraqi" and "al-Durr al-Mukhtaar"