In
Red Rackham Treasure, big crabs (Crab = Cancer) pinch the Thompsons' toes (p.25-C1) on the 21st of June, as can be calculated via a reverse chronology since the 23rd of July (p.56-A1), when the 'Sirius' arrives back home.
see
http://en.wikipedia.org/wiki/Cancer_%28astrology%29 (for Cancer Sign, the wikisite in French gives 21 June to 22 July).
Maybe there are more Zodiac Signs to be found in Tintin?
Might be worth mentioning
Sirius is a star related to the 'dog days' (Fr. canicule), a hot period that occurs mainly in July!
see
http://en.wikipedia.org/wiki/Dog_Days
PS: thanks admin for capitalizing 'Sign' in this post's title!