关于nextday与thenextday的区别

新概念2的第57课有这么一句话Thewomamwalkedoutoftheshopangrilyanddecidedtopunishtheassistantnextday.

为什么这里用nextday而不用thenextday,两者有什么区别.原文是

Awomaninjeansstoodatthewindowofanexpensiveshop.Thoughshehesitatedforamoment,shefinallywentinandaskedtoseeadressthatwasinthewindow.Theassistantwhoservedherdidnotlikethewayshewasdressed.Glancingatherscornfully,hetoldherthatthedresswassold.Thewomanwalkedoutoftheshopangrilyanddecidedtopunishtheassistantnextday.Shereturnedtotheshopthefollowingmorningdressedinafurcoat,withahandbaginonehandandalongumbrellaintheother.Afterseekingouttherudeassistant,sheaskedforthesamedress.Notrealizingwhoshewas,theassistantwaseagertoserveherthistime.Withgreatdifficulty,heclimbedintotheshopwindowtogetthedress.Assoonasshesawit,thewomansaidshedidnotlikeit.Sheenjoyedherselfmakingtheassistantbringalmosteverythinginthewindowbeforefinallybuyingthedressshehadfirstaskedfor.