See Halachically Speaking Volume 3 Issue 16 page 5:
When Lag B’omer falls out on Sunday (as it does this year) shaving etc. is permitted on Friday because of kovod for Shabbos. The reason why it is kovod for Shabbos is because by the mincha before Lag B’omer one does not say tachnun, so technically shaving would permitted on Shabbos, but since one can not do so, the allowance is pushed back to Friday. One may not shave etc. on Motzei Shabbos when Lag B’omer falls out on Sunday. Furthermore, many say that shaving etc. in the above situation is not permitted on Thursday night [Thursday night is technically Friday] if one has time to do it on Friday. [...]