Don’t know. Read something nice about Tu B’shvat from R. Avigdor Miller. It is like Gan Eden because we eat fruit that are available without b’zeias apecha toachal lechem.
Its not a matter of no tachnun in order not to say vyihi noam (when rosh chodesh is mid week we say viyhi noam), Its when yom tov falls out in mid week do we not ssay viyhi noam.