The Mishna (Meg 6b) writes that the only difference between Adar I and Adar II is that the reading of the megillah and matanos la'evyonim must be done in Adar II. The implication is that the day of 14 Adar I still counts as a holiday and should be celebrated as such, e.g. have a seudah. Tosfos, however, rejects this idea. The gemara writes that the two 14 Adar dates are identical with respect to hesped and ta'anis, meaning you cannot fast or deliver a eulogy on either one. Sounds like only with respect to the issur of hesped and ta'anis are the two dates identical, but not that one is required to have a full celebration on 14 Adar I just like 14 Adar II.
Tosfos seems to be splitting hairs a little too finely. How do we know that one cannot deliver a hesped or have a ta'anis on Purim? Because, as the gemara darshens (5b), the day is described as one of "mishteh v'simcha," partying and celebration. How can one say that a full holiday celebration is not required, i.e. 14 Adar I is not a day of "mishteh v'simcha," but at the same time say ta'anis or hesped are not permitted? Either it is a day of mishteh v'simcha or it's not, but you can't straddle the fence. Something to look into over Shabbos.