Lohengrin: year from book, date from Times: http://infotrac.galegroup.com/itw/infomark/164/784/51332809w16/purl=rc1_TTDA_0_CS50518778&dyn=4!xrn_15_0_CS50518778&hst_1?sw_aep=rtl_ttda dated Thurs 26th, despatched, 21st (Sat) Lohengrin done on Friday i.e. 20th Also NYT http://query.nytimes.com/mem/archive-free/pdf?res=9505E4D6103BEF33A25751C1A9679D94629ED7CF
Mastersingers Advert Jun 30 gives dates of festival but not of individualoperas
http://gallica.bnf.fr/ark:/12148/bpt6k199971.image.r=Bayreuth.langEN.f276.tableDesMatieres Revue wagnérienne 1887/02 (T3)-1888/01. p.261
Tristan July 25 probable - from Times advert festival started on 23, Tristan in 26 July (pub 27) dispatch from Germany mentioned as given "last night". News report on Liszt's death mentions he attended that night which fits with being premiere. http://gallica.bnf.fr/ark:/12148/bpt6k19996p.image.r=Tristan.langEN.f189.tableDesMatieres Revue wagnérienne 1886/02 (T2)-1887/01. p178
Tannhauser July 22 likely despatch July 24, pub 29 refers to performance "last Wednesday" i.e. 22nd. advert indicates festival started on 19th. Despatch on 21 says T "is being looked forward to" Confirmed vua NYT http://query.nytimes.com/mem/archive-free/pdf?res=9A06E2DF1F3BE533A25752C1A9679C94609ED7CF
Dutchman No info in Times.
http://query.nytimes.com/mem/archive-free/pdf?res=9402EFDE163DEF3BBC4B51DFB4678382669FDE Page 4